Zero Standby Current Timer

This is a timer for controlling LED lights powered from a solar-charged battery. It was built to illuminate a passageway between two terraced houses, which is used to move the rubbish and recycling bins from the gardens of both houses to the street on collection days. Power is supplied from a battery charged by a solar panel. To guard against battery wastage due to leaving the lights on, it was desired to have a time switching arrangement where a single push of the switch gives a fixed duration of light before switching off automatically.

The first intention was to use a mechanical time delay switch, but this was found to be a rather costly solution.

The lights themselves were modified from 3-LED, battery powered push lights obtained in packs of two from a pound store (I frequent such places, searching for anything with white LEDs to use in my experiments!) The modification was simple but fiddly; entailing some track cuts to rewire the LEDs from a parallel to a series circuit and bypass the switch, and a resistor change. The timer itself was built inside an MB2 plastic enclosure with a pushbutton switch.


Circuit operation is as follows:

When SW1 is operated, C1 charges quickly via R1. Current flows through CR1 into the LEDs and the rest of the circuit. IC1, LM2903 is a dual comparator (only one half is used). The voltage at pin 3 rises higher than the voltage at pin 2, and so the output pin 1 goes high. R6 is required because the 2903 has open-collector outputs. It should be omitted if a dual op-amp such as LM358 is used instead. TR1 turns on and RY1 pulls in. By the time SW1 is released, the LEDs and circuit are now receiving power via RY1. The voltage at pin 3 falls until it drops below the voltage at pin 2, causing TR1 and RY1 to turn off. The voltage on pin 2 rises, to ensure TR1 will not turn on again before the relay contacts break fully.

The timeout is set by R2, R3 and C1. These values give approximately a 3-minute timeout with RV1 about a quarter of a turn clockwise, which was found to be plenty of time to complete the bin run. If more time is required, SW1 can be pressed again to restart the 3 minutes.

Note that CR1 and SW1 have to withstand the full startup current of the load (here, four series strings of three LEDs at 30mA, so well within ratings). To overcome this limitation, use a double pole relay: one pole switching the IC and coil, and the other pole switching the load. Alternatively, use a second relay (connected as the load on the timer relay) to switch the main load.

3 Responses to “Zero Standby Current Timer”

  1. electromozzo says:

    Thanks for taking the time to write in. We aim to update out blog at least on a weekly basis.

  2. MishaPowerauto says:

    Are you from San Diego? – cool!!!!

  3. AJS says:

    Are you from San Diego?

    No, try again — you’re only a few megametres out.

    What country is San Diego in? And what country is this blog hosted in? I’ll give you a clue, look at the last two letters of the site address.