A $20 Power Failure Timer

I have a second home that is ~90 minutes from my primary residence. It’s in an area that is prone to frequent power outages, so you’re never quite certain when you arrive how safe the food in the fridge might be to eat. I wanted a simple way to know if the power had gone off while I was away and, most importantly, how long it was off.

I originally planned to use an Arduino and RTC module, but decided on a simpler solution – a basic kitchen timer with a “count up” function. By wiring a relay across the Start/Stop button, I could easily track cumulative time. Total cost was about $20.

Parts

Taylor Kitchen timer - $10

http://www.amazon.com/Taylor-584721-Digital-Timer-...

120VAC SPDT Relay - $6

Enclosure – Electrical junction box & cover - $2

Power cord (scavenged from a discarded appliance)

Thin wire (22 ga should work)

NOTE: AC Mains power is dangerous! If you don’t feel comfortable working with it, use a low voltage relay (5VDC or 12VDC) and power it from a “wall wart” type of transformer. Using low voltage would also eliminate the need for the case – just hot glue the relay to the back of the timer. I used 120VAC because 1) I had a relay handy and 2) I didn’t want a transformer hogging a power outlet.

The schematic shows how simple this is. I added wires to the timer that are connected to the relay contacts. Whenever the relay changes state, it “presses” the Start/Stop button on the timer and tracks cumulative time.

Most cheap kitchen timers only count down, but I needed on that also counts up. I also needed one that didn’t care if you held down the Start/Stop button (more on this later). The one I chose has a nice, big display, runs on a single AAA battery, and has a case that is easy to open. Plus, it has "Chrome Accents"!

Opening the case is easy. First, remove the two screws in the back and use a thin bladed screwdriver to pop open the bottom of the case. Inside, remove the small screws that hold the PC board to the front of the case. The LCD display may pull away from the PCB, so be careful to not damage the soft silicone contacts on the LCD.

Solder 12” lengths of thin wire across the contacts of the Start/Stop button, being careful to avoid damaging the conductive metal contact. I scraped away a bit of the protective green coating so that the solder would adhere to the copper trace on the PCB. I also added a wire from the “Clear” button for future use.

Carefully reattach the PCB to the front of the case and route the wires out through a hole in the back. Screw it all back together and test that the buttons still work. Also confirm that the wires will start and stop the timer by briefly touching them together. If it all works, congrats! You’re ready for the next step.

I used a 120VAC SPDT relay to start and stop the timer. A SPDT relay has 2 contacts – when the relay is not energized, one is Normally Open (NO) and the other is Normally Closed (NC). When power is applied, they change state – the NO contact closes and the NC opens. These NO & NC contacts are connected in parallel to the Start/Stop wire we added to the timer. I tried a few different relays before I found one that worked well - the first couple I tried reacted so quickly that the timer didn't read the state change as a button press.

I enclosed the relay in a plastic junction box and used heat shrink tubing to protect against shorts. The wire from the relay contacts exit the case and connect to a terminal strip, further removing the low voltage side from the high voltage side.

NOTE: AC Mains power is dangerous! If you don’t feel comfortable working with it, use a low voltage relay (5VDC or 12VDC) and power it from a “wall wart” type of transformer. Using low voltage would also eliminate the need for the case – just hot glue the relay to the back of the timer. I used 120VAC because 1) I had a relay handy and 2) I didn’t want a transformer hogging a power outlet.

Use a blank outlet plate to close the box and keep all the high voltage stuff inside. Connect the wires to the relay contacts. Polarity doesn't matter, but note that one wires goes to the "Common" on the relay while the other is connected to both the NO & NC contacts via a short jumper. Connect one of the wires directly, but leave the second wire disconnected - you'll need to break the circuit briefly after power is applied. Instead of the loose wire, you could add a switch (a NC pushbutton would do the trick) instead.

By using a longer length of wire, the relay could be located out of sight and the timer could be mounted in a more visible location.

When power is first applied, the NO contact closes and starts the timer. Remember what I said about the timer not caring how long you held down the Start/Stop button? This is where that becomes important - the timer’s Start/Stop button only operates one time per press, meaning that it ignores the fact that the contact remains closed as long as power is applied.

To use the timer, first be sure to set it to “Count Up” mode. Plug it in to the AC outlet - the timer should beep and start counting up. Briefly disconnect and reconnect the second wire to stop the count, then press “Clear”. The timer should read “00:00:00”. Now, whenever power is interrupted, the timer will start counting up. You'll need to briefly disconnect second wire to reset the timer after a power outage.