Hacking a plug-in timer for battery operation
(Last modified 15 Feb 12)

I needed a battery-operated timer for an LED project, but all I could find at the local hardware store were the cheap, plug-in-the-wall timers for AC appliances.  Except for the AC power, the timer had everything I needed; LCD screen, lots of programmable features, RTC built in.  So I took a couple of them home to see what I could make from them.

The device I chose is a Westek 1-Outlet Digital Timer with 20 (!) programmable on/off events per week (not that I'll ever use that many).  Check the pictures below for details.

New timer and electronic guts

Here is a new, unopened timer, alongside the guts from the timer I did this hack on.  Notice that the timer is running, even though it is unplugged from the mains.  The timer is being powered by an 80 mAH, 1.2 V NiMH battery (green and silver disc on the PCB).  Circuitry on the PCB will recharge the battery any time the device is plugged into an outlet.  Since the timer probably hasn't been connected to mains in weeks, you can see the timer and display electronics draw very little power.

The black cube on the right end of the PCB is a 24 VDC relay that switches mains power to whatever is plugged into the timer.  This relay in turn is switched on/off by a transistor (hidden by the battery).

Backside of the display board

Here is the backside of the display board.  The three white wires provide GND, 1.2 VDC, and an output from the board that controls the transistor mentioned in the above paragraph.  This output is 0 VDC to turn the relay off and 1.2 VDC to turn the relay on.

Case ready for battery holder

In this photo, I've taken a Dremel tool to the inside of the case that originally held the power PCB.  I've removed all of the molded plastic that held the PCB and the prongs to the AC plug, leaving a gap large enough for a single AA battery holder.

When you use a Dremel on the case, be sure to wear a dust mask and use good ventilation!  The dust and fumes from this operation are nasty and cannot be good for your lungs.

Wired for battery operation

Here I've added a Radio Shack AA battery holder, a AA NiMH battery, a PN2222 transistor, and a 1K ohm resistor.  The transistor and resistor are stuck to the back of the display board with double-sided foam tape.  The battery holder is stuck to the bottom of the case with double-sided foam tape, as well.

The green and black wires are routed through a slot in the side of the case that originally held one of the prongs for the AC plug.  I tied a knot in the two-wire cable to act as strain relief after the unit is assembled.

All done!

All done!  I've reassembled the unit, then used the timer's instruction sheet to set the current time and day.

The electronics
There isn't much to the electronics.  Here is a schematic of the hack:

Schematic of the hack

I connected the output wire from the display board (middle wire in the group) to one end of a 1K ohm resistor, then hooked the other end of the resistor to the base of a PN2222 NPN transistor.  (Yes, the schematic shows 2N2222A; either will work.)  I hooked the ground wire from the display (near the negative lead of the small electrolytic cap on the back of the display board) to the emitter of the transistor.  I hooked the positive wire from the display (near the positive lead of the cap) to the red lead from the battery holder.  Finally, I hooked the black lead from the battery holder to the ground wire from the display.

At this point, I plugged in a AA battery and the timer's display lit up and started tracking the time.

With the battery unplugged, I hooked a long black wire to the ground wire from the display.  I also connected a long green wire to the collector of the PN2222 transistor.  These green and black wires are the control output from my hacked timer; I routed them outside the case through one of the slots left over from the AC power plug.  I reinstalled the battery, put the electronics back in the case, closed it up, and I'm done!

To use this device as a battery-powered timer, you will need to think of it as a 555 timer, but fancier.  The 555 timer has an open-collector output on pin 3; this modified timer also has an open-collector output (the green wire).  To control a load, you need to hook the negative lead of your load (say, a relay) to the green output wire and hook the positive lead of your load to an external power supply, such as a battery or solar panel.  Then hook the negative lead of your power supply to the black ground wire.  Like this:

Hooking up a load

Next, use the timer to turn the load on or off.  As a quick test, you can use the front-panel buttons to step through the options until you see ON blinking.  If you then press the SET button, the load will come on.  Now step through the options again until you see OFF blinking.  Press SET and the load will switch off.

You may not be able to get exactly this model, so you will probably have to use these instructions as a general guide.  I suspect that if you can find a timer that is running while in its blister pack on the store's wall, you should be able to make this hack work.

The PN2222 transistor can be just about any NPN switching transistor.  Be sure to check the data sheet for your transistor, to get the correct leads for emitter, base, and collector.  If you hold a PN2222 transistor so the leads are pointing down and the flat side of the transistor is facing you, the leads (left to right) are emitter, base, and collector.

The original 80 mAH battery was recharged from the mains whenever the timer was plugged into the wall.  The AA battery I used here will basically get shelf life (up to three years for the low self discharge NiMH batteries).  However, if you want to make sure the battery "never" runs down, you can add a small solar panel battery charger to the unit.

To add a solar charger, bring out a separate ground lead (same connection point as the black wire in the output cable) and a red wire that is hooked to the red wire of the AA battery holder.  You can treat these two wires as if they were a NiMH battery that you want to hook to a solar panel charger.  Be sure that the voltage applied to these wires never exceeds 1.4 VDC.  Additionally, provide some kind of protection against overcharging the battery; limit the current to no more than C/10, where C is the discharge capacity of the battery (2600 mAHr, in my case).  Finally, be sure to include a blocking diode between the solar panel and the battery connection, to keep the solar panel from draining the battery when the panel is in the dark.  For excellent discussions on solar panel battery chargers, check this page and this page.

Although I show a NiMH battery here, you could also modify this circuit to use a solar-charged supercap.  You would need to replace the battery with the appropriate supercap and add a suitable charging panel, but you would end up with a timer that would basically run forever.

I can see this timer being used to control all kinds of low-voltage projects, such as LED garden displays and artwork, remotely located data loggers, or long-term photo equipment.  In particular, you can control Arduino (or other MCU) projects that don't have a built-in real-time clock but need to wake up now and then, on a schedule, and perform some action.

If you try this project, please drop me an email.  I'd like to hear what other timers work for this hack.