The circuit is designed to keep the power to the car radio for a couple of seconds after the ignition shuts off. You may ask — why? I often wait in the car with the engine turned off, but the ignition key is in position 1, so I can listen to the radio. Unfortunately, to start the engine again, I have to turn the ignition off completely (position 0). It annoyed me that the radio stopped playing for a moment and the automatic antenna started to retract, only to extend again. This was particularly frustrating when listening to a CD, as it took a while for the player to find the right track again. Additionally, such a short pause caused extra wear on the CD and antenna mechanisms. I decided to create a simple circuit that would keep the radio control voltage for about five seconds to mitigate this problem.
Required components
- Any NPN transistor for example BC546, BC547
- resistors: 22 kΩ, 10 kΩ, 100Ω
- electrolytic capacitor 220µF
- any two small-power diodes, for example 1N4148
- 12 V relay, at least 5 A switching capacity
- some plastic box, I used DAT cassette box
- 4-way terminal block
Build
The circuit is dead simple.
When the ignition is turned on, the ING terminal is kept at +12 V.
When the ignition is turned off, capacitor C1 is gradually discharged through resistor R3.
When the voltage drops to a sufficient level, the transistor cuts off the current flowing through the relay.
The relay’s output is connected to the radio’s control input.
This way, the radio is still powered for a few seconds after the ignition is turned off.
The capacitance of capacitor C1 determines the relay’s hold time.
It should be noted that this time also depends on the battery voltage and the characteristics of the transistor and the relay.
The circuit is not precise, but completely sufficient for this application.
In my case, the delay was about 7 seconds.
The device was installed in my Mercedes-Benz W124 and works perfectly. When the ignition is turned off, the radio still plays and the automatic antenna is still extended.