Incandescent Lamp Dual Timer (ART189E)

The circuit works as follows. When S1 is closed, capacitor C2 charges through R1 and R2 until Vp is reached. At this moment, the IC 1-a output goes to a low logic level, and C3 begins to charge through R3 and R4 until VI1 is reached.

During the first capacitor charge (C2), IC1-b, c, and d outputs remain at the low level, and the SCR is off. During the second time delay, when C3 is charging, ICl-b, c, and d outputs remain high, and the SCR is triggered on, supplying the lamp from the ac power line.

As soon as the capacitor C3 is charged to Vn, ICl-b, c, and d outputs go to a low logic level again, and this causes the SCR to trigger off. To set a new time delay, you have to discharge C2 by pressing S2.

Remember that the SCR is a half-wave control to ac loads. A diode bridge can be used to give you a full-wave control. Do not use inductive loads or fluorescent lamps in this circuit.

A schematic diagram for the Incandescent Lamp Dual Timer is shown in Fig. 1.

Figure 1 – Schematic diagram of the dual timer

The positions of the polarized components [power supply (dc) and electrolytic capacitors must be correct. The SCR must be mounted on a heatsink.

With a 1,000 µF capacitor for C2 and C3, and 4,700,000 ohm potentiometers, the total time delay is up to 90 minutes. Current drain from the dc power supply is very low, extending battery life to several weeks.