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How To Keep Your Radio On When You Start Your Car July 29, 2010

Posted by Robert Harder in Utility.
Tags: , , , , ,

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 noticed that on some browsers, the fine lines of the schematic don’t show very well. You may want to click on this picture to enlarge it.)

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.

Mystery Relay: Daytronic model 9398


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.

Update: 2015

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.

Cascading Relay Circuit Board

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.


1. sam - December 5, 2012

Robert, I have the same problem, do I need the relay? I’ve read other examples without the relay, what would the difference or effecrtiveness be without the relay?

2. Robert Harder - December 6, 2012

@sam The capacitor and relay work in concert to keep the radio powered for a time after the accessory power turns off. I suppose if you had a bigger capacitor, you could keep the radio powered without the relay, but that would be a big capacitor, and I don’t know how your radio might respond to a slowly dropping voltage. With the capacitor/relay combination, the radio sees a constant voltage until the relay shuts off.

That said, I have a different vehicle now with this same circuit, and the radio is so sensitive that it still senses a change when the car starts, so this whole circuit doesn’t work for my new radio. =(

3. Miikka K - March 10, 2013

I just wanted to thank you, I already had 10 x 4700uF capactiors in my shopping bin to get this done with electromechanical relay, but then I stumbled upon your post. You made my day, I was really reluctant to buy & install that many big capacitors after watching cap explosion videos in youtube 😀 haha. Well have a nice day!

4. Hip Hop producers in LA - October 17, 2014

It’s really a cool and useful piece of info. I’m happy that you just shared this useful information with us.
Please keep us up to date like this. Thank you for sharing.

5. Isaac Watson - December 10, 2014

I hate reading long articles, simply as i have got a small amount
of dislexia, but i really loved this post

6. josef - February 9, 2015

Very useful. I just want to confirm that the ssr must be a dc-dc relay. Right?

Robert Harder - February 25, 2015

If I’m following you, yes, you want a DC-DC relay, so that the 12V car system can trip the “coil.” Though I suppose it doesn’t really matter if the common, normally open, normally closed pins are “expecting” DC or AC.

7. https://www.youtube.com/watch?v=vjvFOG7ZyFY&index=15&list=PLGYb3eTcr5F351zzdh2ZOfzgFyc6XWC5S|https://youtu.be/vjvFOG7ZyFY?list=PLGYb3eTcr5F351zzdh2ZOfzgFyc6XWC5S|https://www.youtube.com/watch?v=vjvFOG7ZyFY&index=15&list=PLGYb3eTcr5F351zzdh - May 26, 2015

Hello! I’m at work surfing around your blog from my new iphone 3gs!

Just wanted to say I love reading through your blog and look
forward to all your posts! Carry on the fantastic work!

8. Falkor - August 21, 2015

I did this (circa 1988) in my pizza delivery days, and haven’t done it since. After doing a refresher online, found this, and did the same thing on my current BMW.

The stereo now will stay on for an additional 10 seconds (also powers the amplifier too) and also endures the occasional startup reboot. Constant music throughout, and will eventually turn off after the keys are pulled (about 10 seconds). Its a nice feature, and I wonder why manufactures haven’t incorporated this into their head units or cars.

I used a 16v 4700uF cap and it’s a bit smaller than a D-cell battery.
I then tried to build smaller cap-banks, 16v 10,000uF and that’s about the same, I don’t know (yet) the best way to go, whether it be the smaller banks, or the larger single cap, but it’s something i think i will do to all my vehicles (as needed).

thanks for keeping the webpage up!

9. Naaman - December 8, 2015

As long as the radio isn’t ultrasensative to voltage drops, two relays should do the trick. The first – wired normally-closed – connecting the acc wire of the radio to 12v constant power; the second – wired normally-open and grounded by a open car door – will ground the coil of the first NC relay, cutting power to the radio when a door opens.

10. Robert Harder - December 8, 2015

If only my radio behaved so well…

11. Naaman Geist - December 8, 2015

After reviewing my first comment, I realize I’ve gotten this bass ackwards. The first relay should be NC and the second NO. The first powered by ACC, connecting 12v constant power to the radio’s ACC wire. The first relay would be activated/grounded by the second NO relay. The second relay would be powered by 12v constant power and grounded/activated by an open door. So, as long as the door is shut, the radio will receive constant power, regardless of ignition switch position. When the car door opens, the second relay will close, grounding the first NC relay, cutting power to the radio.

12. Naaman Geist - December 8, 2015

Can’t seem to get this straight in my head. The first needs to be NO. Otherwise, the radio would only power back up the the door is shut again. The the second NC will keep the first relay closed until the door opens.

Yes, Robert. Some radios will reboot due to the vehicles starter motor pulling the voltage too low.

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