How To Keep Your Radio On When You Start Your Car
Our 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.
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.
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.
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.
I got smart and made a new version of this that uses a transistor to hold open a small relay which in turn can hold open a great big solenoid relay (if you need that much power). Using the transistor means a smaller capacitor and normal coil relay work fine to hold the circuit after the accessory power turns off. I’ve got tons of those Songle 12V blue relays that work great.
The circuit here in CircuitLab.com also shows where I have an override switch, so I can turn on the radio with the car off.