I need to cut a signal wire after one month from it start working.
So I am looking for timer circuit that it work by small battery make a short circuit for my signal wire after around ( few days more less ) one month.
Electrical – One month timer circuit
controlrelaytimer
Related Solutions
Based on this answer, I think a 555 won't work. You'd need a massive capacitor and a huge resistor, and the accuracy would be terrible.
I'd recommend using the simplest of 8-bit microcontrollers. You'd have to handle the numbers carefully, but a PIC16F84A (or an Arduino, or lots of other similar devices) could do it. I might set up a 1 second timer, and then count 15778463 timer expirations, storing the count in a 32-bit variable. (For Arduino 32-bit variables, see http://www.arduino.cc/en/Reference/Double)
Here's what I would consider: -
In series with the microwave is a bridge rectifier capable of taking the microwave load current and AC voltage. On the DC side of the bridge is a power resistor in parallel with a low voltage relay such as 5V rating (or maybe even lower if you can get one). The power resistor is chosen so that when the microwave takes full load current, there is sufficient voltage across the relay coil so that it activates and turns the heater off.
When the microwave is done cooking, the voltage across the relay drops sufficiently and the relay drops-out and power is restored to the water heater. Here's a 3V relay made by Panasonic. It's got NO and NC contacts rated at 8A up to 250Vac. The order code (209-5597 ids for Farnell a UK electrical supplier so it's probably irrelevant in other parts of the world): -
The relay coil is 90 ohms and activates on (say) 2A taken by the microwave, the resistor and 90 ohm coil have to "drop" (say) 3V. This means coil and resistor together form a 1.5 ohm value. Clearly, as the relay is 90 ohms you can use a 1.5 ohm resistor. The thing to watch on this circuit is the microwave current. If it is (say) 2A max, then the resistor will dissipate about 6W and this is a little on the high side. Any more than 2A and I think I'd recommend a different technique using an opto-isolater built where the relay coil is shown. This could then activate a normal AC relay via fairly standard techniques.
Please let us know what the microwave current is when heating food and what is when just powering its clock - this is important to know for this design to work.
Best Answer
Even the tiny and cheap PIC 10F200 can do this. It comes with a built-in 4 MHz oscillator, which means 1 MHz instruction clock, which you can divide down in firmware to get any length delay you want.
The easiest on this part is to use the timer wrapping as the base event for your clock. This is a 8 bit timer, so that would be every 256 instruction cycles, or every 256 µs. The firmware spins in a loop waiting for the high bit of the timer to change from 1 to 0. It then runs the code to process one more clock tick, waits for the high bit to become 1, then goes back to the top of the loop. You can easily do all the clock tick processing in 256 instructions.
You want to delay about 30 day, which is 2.6 Ms. at 256 µs/tick, you need to wait 10.1 GTicks, or 233.2 ticks. You therefore need at least a 34 bit counter. Since it's easier to use whole bytes, use 5 bytes for your counter.
Initialize the counter to the value for 30 days on startup, then decrement it by one every clock tick. You then check it for being 0, and activate your switch when it is.