How To Keep Your Radio On When You Start Your Car July 29, 2010
Posted by Robert Harder in : Utility , trackbackOur car radio has the annoying habit of forgetting what part of a CD it was playing when you turn it off. On long drives this is a real drag; I’ll leave an audio book playing for my family in the car while I gas up, and when I turn the key from Accessory to Start, the radio turns off and back on again.
I built a cheap circuit that keeps the radio powered up five seconds after accessory power is interrupted, and this keeps the radio running while the engine turns over. The three key components for this circuit are a diode, a capacitor and a relay.
Schematic
Here’s a simple schematic of the circuit. From the car’s accessory power, we connect a diode, and from the diode we connect a capacitor and a relay. When the car’s accessory power is turned on, the capacitor is charged, and the relay is activated (the switch closes). Thus the radio has power. When the accessory power is interrupted, the capacitor continues to power the relay until the capacitor has drained enough that it cannot keep the relay activated. The diode keeps the capacitor’s energy from leaking back into the car’s circuitry.
I should point out that you’ll want fuses in relevant places. I left them out of the schematic for the sake of making the pertinent parts more clear.
Parts
Someone smarter than I could probably calculate exactly what capacitor size and relay specs you’d want to hold the relay active for so much time, but I just experimented. I found that I needed a solid state relay because the mechanical, electromagnetic kind drained the capacitor too quickly. Even among the solid state relays I tested, some drained the capacitor faster than others.
- Diode. I used a simple “signal” diode that I had in some old Radio Shack kit.
- Capacitor. I think it was 35V, 1000 micro farad.
- Solid State Relay. This one is a mystery. It says “Daytronic solid state relay model 9398,” but I cannot find out anything about it online.
Mystery Relay: Daytronic model 9398
Extensions
If your relay doesn’t support enough current, as my 10A relay did not, you can use that relay to power the coil of another, bigger relay. In my case I got a traditional electromagnetic relay with a 70A limit, or something ridiculous like that, to power an amateur radio.
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