FBI siren with flashing light

Posted by Circuit Labs on Monday, November 24, 2008

This ultra-simple circuit will produce the familiar sound of sirens used by US police cars on emergency calls. A small lamp will also flash synchronously with the siren sound. The circuit is capable of powering loads greater than 1 A for one or more lamps or a powerful loudspeaker, the kit producing quite a bit of noise and light.

The circuit is built from two astable multivibrators, in this case the familiar 555 of which two are present in an NE556 case. Of course, you are free to use two 555s if that suits you better. Both timer ICs are configured to operate as astable multivibrators.


The first timer is configured with R1, R2 and C2 to supply a rectangular signal of about 2 Hz at pin 9. The lamp is switched on and off by way of power transistor T1. The second 555 is configured using R4, R5 and C5, and supplies a square wave at pin 5 that drives the loudspeaker. The toggling voltage at the output of the first timer (pin 9) causes electrolytic capacitor C3 to be partly charged and discharged, periodically, via resistor R3. C3 is connected to the control input of
the second timer (pin 3), causing it to work as a VCO (voltage controlled amplifier). The upshot is that the frequency of the square wave applied to the loudspeaker rises and falls periodically, rendering a good imitation of the wailing sound of the US police car siren (we hear too often in movies).

The small number of dead-standard components used enables this circuit to be built on Veroboard without problems.

Author: Arthur Schilp

(Elektor Electronics Magazine – 2006)

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Energy recovery

Posted by Circuit Labs on

u060372-11Energy is becoming more and more expensive, and so we are always on the lookout for ways to save energy in circuits. The author has decided to look at how to recovery energy from a relay switching circuit. If a relay is driven by a transistor switching stage it is usual to connect a flywheel diode in parallel with the coil to short out the back EMF produced when the relay current is switched off (Figure 1).

If an LED is wired in series with the flywheel diode (Figure 2) it will flash every time there is an inductive spike with the transistor turns off. The duration and brightness of the flash (and indeed, whether the spike is large enough to destroy the LED!) depend on the rate of change of the current in the relay coil and its inductance:

ui = –L di / dt

u060372-12So far we have not actually recovered any energy. Figure 3 shows a theoretical design where the energy stored in the relay coil is recovered so that it can be used to supply a (low-power) circuit.

The greater the inductance of the coil and the more frequently it is switched, the more energy is stored in capacitor C. The zener diode (in series with the flywheel diode) limits the maximum voltage to which the capacitor can be charged. Measured relative to ground the open-circuit voltage at point A is the sum of the capacitor voltage due to the recovered energy and the supply voltage. In particular, the voltage at point A is higher than the supply voltage.


The author would be interested in discussing these ideas further with readers. His e-mail address is

Author: Peter Lay

(Elektor Electronics Magazine – 2006)

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Opamp With Increased Output Current

Posted by Circuit Labs on

u060373-2-11Integrated opamps offer advantages such as ease of use, good price-performance ratio and small physical size. However, they seldom have an output current drive capability of greater than about 12 mA, and so they are not suitable for use in 20 mA current loop applications, for example. One solution is to add a driver stage with the necessary output power, comprising perhaps two to four transistors and a number of other components.

This design takes up board space and is relatively expensive, tending to offset the advantages of the integrated device. An alternative possibility is to boost the output drive capability by connecting opamps in parallel. The output current will then be approximately proportional to the number of opamps. Instead of a single opamp a dual or quad device is used to achieve greater output power. u060373-2-12

The idea is shown in Figure 1. The output of the first opamp is connected to the input of a further noninverting opamp stage as well as being connected to the output of the circuit via a resistor. The first opamp thus drives the second non-inverting amplifier which provides all the output current of the circuit as long as that remains within its normal capability.



As the output current demand increases the second opamp will reach the limit of its drive capability. Its gain will then fall off and a voltage difference will develop across its inputs. The first opamp will then start to deliver more and more current to the output via the resistor, and the sum of the output currents of the two opamps thus flows through load resistor RL.


By adding another resistor we can compare the current contributions from the two opamps (Figure 2). The complete circuit with two opamps is shown in Figure 3. The principle of the circuit can be extended to more opamps with their output currents being added together (see Figure 4).


Author: Klemens Viernickel

(Elektor Electronics Magazine – 2006)

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Energy saver for relays

Posted by Circuit Labs on

When dealing with relays we distinguish between the pull-in voltage and the hold voltage. Depending on the type of device, the latter is from about 10 % to 50 % lower than the former. This means that once we have safely pulled in the relay armature we can drop the coil voltage to its hold value, thus reducing the power dissipated. The simple circuit shown here does just that: it consists of a parallel combination of an LED, an electrolytic capacitor and a resistor, together placed in series with the relay coil. As well as saving energy by reducing the operating current of the relay and increasing its operating life the circuit also has the advantage of providing a status indicator in the form of the LED.u060373

The author has used this circuit with practically all types of relay, with various rated currents and voltages. The recommended component values are as follows:

• The electrolytic capacitor should have a value between 100 μF and 1000 μF with a working voltage of 6.3 V, depending on the rated current of the relay coil.

• The resistor value should be between 10 Ω and 1 kΩ so that in the active state a current of 20 mA flows through the LED.

• A standard green or yellow LED with a rated forward current of 20 mA should be used. When using relays with a very low coil current low-current LEDs may be substituted. Add a zener diode in series for higher coil voltages such as 24 V or 48 V.

Author: Klemens Viernickel

(Elektor Electronics Magazine – 2006)

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Accurate timebase

Posted by Circuit Labs on

This circuit demonstrates how easily a timebase can be designed using a minimum number of components. This circuit consists of no more than an IC and an oscillator to which a few connections and decoupling parts are added. Use is made of a single IC from the 4000 series, the type 4521 oscillator / counter with 24 steps. Here we only use the counter section of the IC.u060573[5]

The IC is supplied with a clock signal by an oscillator module. The clock signal is divided down by the 4521 to obtain certain values. Discrete frequencies obtained in this way are available on the counter output pins of the 4521. For example, pin 10 (Q18) supplies the clock frequency (applied to pin 6) by a factor of 218 or 262,144. Likewise pin 1 (Q24) divides the input signal by 224 or 16,777,216.

By using a clock frequency of Accurate timebase 2.097152 MHz the following timebase frequencies are obtained at the output of the 4521:

• Pin 10 (Q18): 8 Hz;
• Pin 11 (Q19): 4 Hz;
• Pin 12 (Q20): 2 Hz;
• Pin 13 (Q21): 1 Hz;
• Pin 14 (Q22): 0.5 Hz;
• Pin 15 (Q23): 0.25 Hz;
• Pin 1 (Q24): 0.125 Hz.

In case other frequencies are required, a different crystal oscillator module should be selected. For flexibility, an IC socket is recommended so modules can be exchanged quickly. Alternatively, modules may be connected to the 4521 by means of a selector switch.

To keep the circuit as versatile as possible, the outputs of the 4521 counter may be made available for connection to the outside world by way of a pinheader. For the sake of convenience, the pinheader also supplies +5 V and GND.

Author: Thomas Pototschnig

(Elektor Electronics Magazine – 2006)

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PCB pin insertion tool

Posted by Circuit Labs on

u060573The humble PCB pin is used not only to connect wires to PCB tracks, but also for test and measurements points on circuit boards. Despite their mechanical simplicity, PCB pins are surprisingly difficult to mount and solder. Frequently encountered problems (admit it!) are burnt fingers while soldering the pin, and pins dropping from the board when the board is turned, or, worse, when they’re being soldered. Try to keep it in place with your finger — too hot to touch — look for pliers — pin dislocated — and so on.

It’s a good idea to drill the holes for the PCB pins a fraction smaller than the pin diameter — that way, the pin remains in place when you’re ready to solder it. However, that also requires a bit more force to push the pin into position and a tool is then useful to prevent injuring your fingers. Pliers may be less suitable, especially if considerable force is required on the pin. To the best of our knowledge, no commercial tool exists for the purpose. No problem, let’s make a PCB pin insertion tool ourselves.

A discarded round file or screwdriver is great. Cut or grind the shaft as straight as you can and drill a hole in the end so the ridge on the PCB pin is secure against the surface of the tool. The PCB pin is inserted into the tool and held in place with a nail or fingertip. The tool handle provides a secure grip and allows considerable insertion force to be applied. If an old file is used, it is recommended to cover the remaining section of the steel surface with heat shrink tubing. This will prevent injury to your hand.

Author:  Luc Lemmens (Elektor labs)

(Elektor Electronics Magazine – 2006)

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Multimedia RIAA Preamplifier

Posted by Circuit Labs on

Even if a large number of album titles once available on vinyl are now, little by little, being proposed as CDs, not all are available and far from it. You may have treasures in your collection that you would like to burn on CDs. First, preserving a CD is easier than preserving a vinyl record, and second, we have to admit that turntables are disappearing, even on fully-equipped Hi-Fi systems. From a point of view of software and PCs, converting from vinyl to CD is not a problem. A large number of programs, whether paid for freeware, are available to re-master vinyl records with varying degrees of success and to eliminate pops, crackles and other undesirable noises. All of these programs work with the sound card of your PC and that, admittedly, is where the problem starts. Most high-quality turntables are equipped with a magnetic cartridge which typically delivers just a few mV. The cartridge signal requires a correction of a specific frequency, called RIAA correction.

If our older readers will perfectly recall what RIAA is all about, others from the CD generation may not know what the acronym RIAA stands for, guessing it may have something to do with illegal downloading of music on the Internet. For mechanical reasons related to the vinyl engraving procedure, high-boost frequency correction is carried out while respecting a very precise curve defined a long time ago by the RIAA (Recording Industry Association of America) and, which therefore, quite naturally, was baptized RIAA correction.


Reversing the correction is the role of to the preamplifier for the magnetic cartridge. Since this correction boosts the lowest frequencies, such a preamplifier is very sensitive to all undesirable noises, hums, including, of course, the one coming from the 50-Hz (or 60-Hz) mains power supply. It is important to take that into account while making this project which must be done carefully with respect to grounding and shielding. The schematic of our preamplifier is very simple because it uses a very low-noise dual operational amplifier. Here the NE5532 is used, whose response curve is modelled by R7, R8, C8, and C9 (or R14, R15, C13, and C14 respectively) in order to match the RIAA correction as closely as possible.

The input has an impedance of 47 kΩ, which is the standardized value of magnetic cartridges, and its 1,000-Hz gain is 35 dB which allows it to supply an output level of a few hundred mV typically required by for the line input of a PC sound cards.

The connection between the cartridge and the input of the amplifier requires shielded wiring to avoid the hum problems discussed above. Likewise we recommend fitting the assembly in a metal housing connected to the electric ground. With respect to the power supply, three solutions are proposed: If you are a purist and you want to rule out any noise whatsoever, you will utilize a simple 9-V battery. Then, the components outlined with a dotted line will not be useful. Since the circuit only uses a few mA, such a solution is acceptable unless your collection of vinyls is impressive...

If you desire a more elegant technical solution that might sometimes cause more undesirable noise on the signals, you may want to wire up the components within in the dotted lines and you can steal the 12 V positive voltage available from your PC. A Y-connector inserted on the power supply of one of the internal drives or peripherals will work very well for that.

Finally, you may also use a mains adapter set to 12 V and connect it to the +12-volt point of the drawing in order to benefit from additional filtering, which is not a luxury for some.

Author: Christian Tavernier

(Elektor Electronics Magazine – 2006)

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Miller Capacitor

Posted by Circuit Labs on

060075ukThere are amplifier circuits that have capacitance between the input and output. If the gain is positive, this can lead to oscillations. If the gain is negative, another outcome is the result. We can deduce this from the following theoretical circuit.

An amplifier with a negligibly low output impedance, an infinitely high input impedance and gain A has feedback in the form of capacitor C (refer Figure 1). The gain A is negative. In addition, input current I, input voltage U and output voltage Uo are also drawn in. The input current I is equal to Ic and the input voltage U is equal to Uc + Uo. Uo in turn is equal to the product A U. From this follows that

Uc = U–Uo = U (1–A).

Substituting into the formula the current that flows through a capacitor, Ic = C(dUc/dt) results in

I = C.dU(1-A)/dt

We rearrange this as  I=(1-A).C.dU/dt

Now we can see that the gain determines the relationship between I and C. C appears to be larger by a factor of (1–A) (note: if A is negative, you can actually speak of a factor 1+A larger).

This is called the Miller effect. The apparent (larger) capacitance is called the Miller capacitance. When designing signal amplifiers you need to take this capacitance into account. We can actually use this Miller capacitance in other ways. If we make A variable, with an adjustable resistor for example, we create a variable capacitor. For this purpose we conceived the following schematic (see Figure 2).


Cm is the apparent capacitor between the input of the circuit and ground. If we connect a signal generator via a series resistor to the input and measure the input voltage with an oscilloscope, we can easily determine the corner frequency.

JFET-opamp A1 is necessary to prevent R1 from appearing in parallel with Cm and affecting the corner frequency. A2 is the actual (inverting) amplifier. The gain of A2 is equal to P1/R1. C is the capacitor which is enlarged artificially. The remaining components only serve to set the operating point of the circuit.

Cb blocks any DC voltages and needs to be relatively large, for example 25 times the maximum Cm. From the test results it appears that Cm is indeed equal to (1+P/R1) C. Cm can be varied with a potentiometer from about 560 pF to 12 nF.

As is usually the case, there are a few limitations in practice. The input signal may not be too large. Otherwise, the AC voltage across Cm will cause clipping at the output of the second opamp. At maximum Cm, the gain of A2 is about 20 times. The peak-to-peak value of the input voltage may therefore not be more than about 1/20 of the power supply voltage. The circuit will always work well for smaller signals, provided the frequency is not too high.

For A1 and A2 we used an LF356 and TL081 respectively. These are mainly used for frequencies not exceeding 100 kHz. Very fast JFET opamps could extend the useful frequency range to applications in the RF-range. For LF applications we could also use a dual
opamp for A1 and A2, such as the TL082.

The value of capacitor C can be changed to suit the application. With opamps of the type AD8099 with a C of 22 pF we can make a (tuning) capacitor with a value from 22 to 440 pF, for use up to 30 MHz. The alternative, a varicap diode that can be varied in capacitance over a range of 20 times (or more) is not used in practice very much any more. Other applications for this circuit are, for example, adjustable LC-filters for audio applications.

Author: Gert Baars

(Elektor Electronics Magazine – 2006)

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Electronic Torricelli Barometer

Posted by Circuit Labs on

Although it does not have the same charm as real mercury barometers with long glass tubes on pieces of carved and polished wood, the Torricelli barometer discussed here is a functional equivalent and electronic replica of the Torricelli barometer.

Actually, rather than displaying the atmospheric pressure on the traditional digital displays, we preferred to reproduce the general look of this respected predecessor of electronic barometers. The mercury tube is, of course, replaced by a simple LED scale which, if not as beautiful, is still less toxic for the environment in case of breakage. As indicated on the drawing, the pressuresensor utilized is a Motorola MPX2200AP.

Click to enlarge...

This circuit is adapted for measuring absolute pressure and has a range well suited for atmospheric pressure. Without entering too deep into the technical details, such sensors deliver an output of voltage proportional not only to the measured pressure but, unfortunately, to their supply voltage as well. Hence they must be powered from a stable voltage which is ensured here by the use of IC1.

Since the output of the MPX2200 is differential and at a very low level, we had to resort to the use of four operational amplifiers IC4.A to IC4.D, contained in one LM324, to obtain levels that can be processed easily. As long as potentiometer P1 is adjusted correctly, this group of operational amplifiers delivers a voltage of 1 volt per atmospheric pressure of 1,000 hPa to the LM3914. Since the atmospheric pressure will be within the range 950 to 1040 hPa at sea level, we need to make an expanded-scale voltmeter with this LM3914 in order to better exploit the 10 LEDs that it can control. That is the role of resistors R7 and R8 which artificially raise the minimum voltage value the chip is capable of measuring.

Consequently, we can ‘calibrate’ our LED scale with one LED per 10 hPa and thus benefit from a measurement range which extends from 950 hPa to 1040 hPa. In principle, you should not have a need to go beyond that in either direction. The circuit may be conveniently powered
from a 9-volt battery but only if used very occasionally. Since this is usually not the case for a barometer, we advise you to use a mains adaptor instead supplying approximately 9 volts.

Calibration basically entails adjusting the potentiometer P1 to light the LED corresponding to the atmospheric pressure of your location at the time. Compare with an existing barometer or, even better, telephone the closest weather station. They will be happy to give you the information.

* After Evangelista Torricelli, 1608-1647, Italian physician who proved the existence of atmospheric pressure and invented the mercury barometer.

Author: Christian Tavernier

(Elektor Electronics Magazine – 2006)

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Modulated Light Barrier

Posted by Circuit Labs on

It’s good fun to keep an eye on all sorts of things in your environment and on the basis of events in this environment to switch, for example, lamps or buzzers. To help with this, the light barrier described here can be used to guard an entrance. You can use it to signal of
someone is walking through the corridor, or to check if the car has been parked far enough in the garage to be able to close the door. The circuit consists of a transmitter, which sends modulated infrared light and a receiver, which recognizes this. The circuit used here is almost insensitive to daylight or fluorescent light and therefore can be used outside.


The transmitter (Figure 1) generates about 1000 times per second, for a period of 540 ms, a burst of 36 kHz. IC1 has been set with C1, R1 and R2 to a frequency of about 1000 Hz. The output of IC1 ensures that IC2 will oscillate about 1000 times per second for a period of about 540 ms. IC2 is set to a frequency of 36 kHz with C2, P1, R4 and R5. The output of IC2 drives the IR LED D1 via transistor T1. C3 and R3 prevent the reasonably high current through D1 from generating too much interference on the power supply rail.


The receiver (Figure 2) is quite a simple design, because IC3 already does a lot of the work for us. When the IC ‘sees’ an IR signal with a frequency of 36 kHz, the output of IC3 will become ‘0’. The transmitter circuit alternates between sending an IR-signal of 36 kHz for 540 ms and is quiet for 470 ms. When this signal arrives at IC3, C4 will discharge via D2. Because the non-inverting input of IC4a is set to 2.5 V, with the aid of R10 and R11, the output of IC4a will be a ‘1’. In the intervening quiet periods of 470 ms, C4 will partially charge via R8, but this is not of sufficient duration to exceed the voltage of 2.5 V. Only when the light barrier is interrupted will C4 charge far enough that the output of IC4a will toggle and become a ‘0’. Because IC4a has an open-collector output, C5 will be immediately discharged and the output of IC4b will become a ‘1’.

With R9 and C5 this signal is stretched to about one second. If you increase the value of R9 to 100 kΩ, then this will become about 10 seconds. R12 and R13 are included to prevent chatter of the output around the trigger point, although  there is not really a risk of that happening in this circuit. Together with R14, the output of IC4b delivers a clean logic signal that we can use for further processing. The quickest way of calibrating the frequency of IC2 to 36 kHz, using P1, is with the aid of an oscilloscope. If you do not have one of those, then point the IRLED D1 at the receiver IC3 and turn P1 so that the voltage on the inverting input of IC4a is as low as possible. Make sure that IC3 during the calibration does not receive too high a signal by placing the IR-LED a considerable distance away or by not pointing directly at the receiver. If this procedure is not that successful then just set P1 to the centre position, this works just fine usually.


You should not have a problem with ambient light with this circuit. If you do have a problem because, for example, there is direct sunlight on IC3, then you will need to place it inside a small tube and point it at the IR LED. In this way no direct sunlight can reach the receiver. If
the IR LED and the receiver are placed too close together it is possible that the receiver will sense light reflected off the walls, even when someone is standing between the transmitter and receiver. In this case the solution is also a short piece of tube for both the transmit LED as well as the receiver (Figure 3). Make sure that the tubes are opaque (paint black or use water pipe, for example). The wires to the IR LED can be several meters long without any problems. Do not place the receiver IC too far from the circuit.

Relate article: Direction Sensitive Light Barrier

Author: Heino Peters

(Elektor Electronics Magazine – 2006)

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