you don't need a microcontroller to make a working timer with enough power to control a contactor. A simple 555 IC can make a timed latch with only a 5V Power Supply, capacitors, resistors, a DIP Switch, an NPN transistor, and an optotriac-triac stage or a relay can do the trick.

the idea is to use the DIP Switch as a resistor selector, so the on time can be selected through a variety of resistors that; using the 555 monostable formula represents the time in a single unit

why not a potentiometer?

because, even when a potentiometer is more precise than a chain of resistors you still need to mark the positions for specific time units (1, 2, 3, 4...). So a chain of resistors can be used to set values that are closer to a specific time.

Capacitors can be used to set the multiplier for each time unit, so if you put a SP2T switch you can get a 1x multiplier and a 10x multiplier, meaning that you can set from 1 minute to 1 hour

Values are set through the 555 monostable formula:

T=1.1*R*C

and with a quick clearing we can find the formula for each resistor.

R=T/(1.1*C)

so if we want a resistor for 60 seconds, with a capacitor value of 220uF we solve:

R=60/(1.1*0,000220)

R = 247933,88 Ohms

247,93K is not a commercial value, but 250k is, so putting 250k in the formula gives a time of:

T=1.1*250000*0,000220

T=60,5s = 1 Minute approx.

Since we are using commercial values a margin of a few seconds is to be expected. we can add up resistors to reduce the margin, but is up to you.

and for a 2200uF capacitor we have:

T=1.1*250000*0,00220

T=605 = 10 Minutes and 5 Seconds

as you noticed, im using a 6 position 12 pin DIP Switch, so i calculated the values from 1 to 6, but you cand expand the values with a larger Switch.

Resistor values are

• 1 = 250K
• 2 = 510k
• 3 = 750k
• 4= 1M
• 5 = 1M + 270k
• 6 = 1,5M

For a Capacitor of:

• 220uF = minutes
• 2200uF = tens of minutes

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Relay should be a 5V 10A one with a 2N5551 low power NPN transistor

And what about the power supply?

your choices are a using a phone charger, you can get 5V with a good amount of current for a low power device such as this one, or you can make a 5V 1A linear power supply but you have to keep in mind that linear power supplies need heatsinks because of power dissipation, or a small transformerless power supply, but is up to you.

this is the schematics for the 5V 100mA linear power supply i made for this:

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as you can see im using the 7805 in its low power version because i think 100mA its enough current.

very messy i know, but it was a prototype so i think its ok, as you can see the 2 terminals between the relay and the bridge are the transformer inputs, so i have to find some space if im going to put everything inside a case (and i couldnt find a 78L05).

is that all?

no!! im planning to use a 7 segment display so the remaining time can be seen, and maybe use a optotriac-triac stage.

but for now its all!