How to use a 555 timer to control a heating strap?

I am using battery to heat a heating strap made from nichrome. And I have manually tested how long it takes the wire to heat to the desired temperature. So i want to know how I can use a 555 timer to off the connection between the battery and heating strap after the desired time. And again back on after after a certain time.

The output of 555 must be connected to a transistor NPN,
The output of transistor must be connected to a Relay,
The output of Relay must be connected to heater.

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You could also use an op amp as a comparator with a thermistor depending on the temperature. That way it will always stay within the set temperature as 555 would not be very reliable if you need a tight temperature tolerance.

EngineerTux said:

You could also use an op amp as a comparator with a thermistor depending on the temperature. That way it will always stay within the set temperature as 555 would not be very reliable if you need a tight temperature tolerance.

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This would be my preferred method of control. If left open-loop (no feedback method), the 555 on/off duty cycle timer could eventually overheat the nichrome to the point of failure, or underheat it to the consequence of a non-useful piece of luke warm wire.

A thermistor may not be able to handle the high temperature of a hot nichrome wire, so you may have to use a thermocouple (two dissimilar metals that produce a small voltage that changes relative to temperature). Generally the output is in the millivolt range, so an op-amp could be used to gain up the signal voltage to a useful level, then set up a second op-amp (or comparator) with some hysterisis (feedback) to keep it from rapidly bouncing the output circuit on and off (think of it as a thermal delay, or window). Take that 5V output, feed a 2N2222 that drives a relay, then the relay closes the circuit to heat up the nichrome wire (andre +1 ).

Or for an even simpler solution, you could buy an inline thermostat with a fixed temperature cutoff and hysteresis window. If you use the comparator method above, you should also design in hysteresis. A Google search should come up with a circuit showing how to implement the comparator with hysteresis.