Author: Hans Zijp
This small circuit can be used to control model railway turnouts operated by AC voltages. A logic level in the range of 5–12 V can be used as the control signal. The coils of the turnout are switched using triacs.
Changes in the logic level of the input signal are passed on by the buffer stage built around T1 and T2. The buffer stage is included to boost the current available at the gates of the triacs. If the input goes high, this positive change is passed through via C1. That causes a positive current to flow through D2 (D2 is reverse biased) to the gate of T3. That triac switches on, and power is applied to the turnout coil. This situation persists until C1 is fully charged. No more current flows after that, so the triac does not receive any gate current and switches off.
If the input is set low, a negative current flows briefly via C1. It can flow through D2, but not through D1. T4 is switched on now, and the other turnout coil is energised. This circuit takes advantage of the fact that triacs can be triggered by negative as well as positive gate currents.
If the turnout coils are energised for too long, you should reduce the value of C1. If they are not energised long enough, increase the value of C1.
The TIC206D can handle several ampères, so it can easily drive just about any type of turnout coil.
You can also use a different type of triac if you wish. However, bear in mind that the TIC206 requires only 5 mA of gate current, while most triacs want 50 mA. That will cause the switching times to become quite short, so it may be necessary to reduce the value of R1.
(Elektor Electronics Magazine – 07/2006)
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