With just three transistors, one relay, and some resistors and capacitors, a single light switch can provide sophisticated control of high-power and low-power lamps.
A brief story behind this idea is as follows:
Last month, I created a simple electronic circuit to answer the following question: Is it possible to use a single mains switch to control two 230 VAC lamps? When the mains switch is first activated, a high power lamp is turned on. After some period of time (approximately one hour), this lamp is automatically powered off and a low-power lamp is turned on instead. I use this solution for my children’s room, because they often forget to switch the light off when going to sleep, and my youngest son doesn't like to fall sleep in darkness.
Note that this is not just another simple high-voltage timer, since I added an RC circuit to be able to switch immediately from one lamp to the other without necessarily waiting for the time-out. We start by turning on the high-power lamp. If we subsequently turn the switch off and then on again, within one second, the low-power lamp is activated. If you wish to turn the high power lamp on again, you do the same thing – turn the switch off and then on again – but this time with a delay greater than two seconds.
No microcontroller; no dedicated integrated circuit; just three transistors, one relay, and some resistors and capacitors. As shown in the circuit diagram (Figure 1), this has been designed to be very low in cost and to use components that are easy to find.
The transformer-less power supply circuit (Cg, R1, R2, D1, D2, C2, DZ1) provides a DC low voltage of 12 V. The full description of this part of the circuit is abundantly commented on the Internet (I recommend reading this application note ). The timing circuit is a passive RC circuit (Rt, Ct), followed by a compound Darlington discrete driver (T1, T2) to control a 12 V 1RT relay. The circuit containing (C10, R10, D3, T3) is the critical part, which allows us to flip from the high-power lamp to the low-power one, simply when switching off then on, quickly. Capacitor C10 must be rapidly discharged through resistor R10, while diode D3 prevents capacitor C2 from also discharging through R10, thereby allowing C2 to discharge slowly through (T2, R6, relay). Meanwhile, as the voltage gate of P-MOSFET T3 quickly goes low, this transistor turns on, and acts as a very low value equivalent resistor in parallel with Rt. Hence, capacitor Ct rapidly charges and activates driver (T1, T2), which then activates the relay.
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