Please read the textbox for more info. The relay is activated approx. 10 seconds after the circuit gets its Voltage - 12 V -. The time is adjustable by putting more 22 uF capacitors in parallel (longer) or take away 22 uF capacitors (shorter). Each 22 uF capacitor adds/subtracts roughly 5 seconds. Please note the following:
1. It is a classical analog timer. The car relay is not activated suddenly with the proper 12 Volt, the voltage and current slowly rise. In the first split second that the contact is made there is no strong contact force between the switch contacts. This could (could) be a problem when you want to switch inductive loads (e.g. a powerful 12 Volt DC motor from say 100 Watt or so).

2. the circuit must be protected completely from moisture and dirt. Moisture and dirt lower the internal resistance values between the components and can give many problems. Especially the circuitry around the base from the BC 547 b is sensitive to moisture and parasitic current.

3. When the car engine is started, switched off immediately, and then started again, it could be that there is still charge on the 22 uF capacitors. This means that the delay time is substantially shorter, compared to the situation that the car stood still for say a few minutes and thus the 22 uF capacitors were able to discharge, giving a completely new, fresh, start time, 10 seconds or so.

4. I don't know how the circuit performs under extreme circumstances, say at - 20 degrees celsius or cold weather. Electrolytic capacitors are sensitive to temperature, it could be that the time delay is not good under very cold circumstances.This can be tested by putting the circuit in the freezer.

5. Always use fuses in car battery circuits, don't forget them or think that they are not necessary. Risk of fire, burn, scorch.

6. use timer capacitors for higher voltages, say in the 40 V-60 V range. They have better timing properties. A cap. from 10 uF at 60 V performs better in a timer circuit compared to a cap. from 10 uF at 15 Volt.

All the way: the circuit can be applied successfully. More circuits in my books on the Lulu website, author Ko Tilman.

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