Seatbelt Warning Light Relay Circuit - Canadian Mustang Owners Club - Ford Mustang Forums


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Old 06-03-2018, 07:41 AM   #1  
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Join Date: Dec 2017
Location: Sudbury
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Seatbelt Warning Light Relay Circuit

I was getting the safety check done on my 67 fastback & found out that the seatbelt warning light needs to work properly. Mine was staying on. SO - I started searching for replacement options... All cost prohibitive. Being an old-school electronics guy, I built a replacement with a 555 timer, a few resistors & capacitor. I changed the lamp to an LED, so that I didn't need an interposing relay. I'm attaching a couple of links: First one is for the circuit I built, the second is for calculating the time constant (how long before the light goes out) You put in the values of the components & the voltage & it will calculate the time for you. You can play with the component values to get the delay you want. I used a 330uf capacitor & a 20k-ohm resistor at 12.7 volts to get a time of 6.6 seconds. I figure that's long enough. On the circuit diagram from the link, the values are different. HOWEVER - the 470 ohm resistor is there to limit the current through the LED, so you don't play with that one or you can burn-out the LED quickly. I had all this stuff lying around, so it didn't cost me anything, but if you have o buy the stuff from an electronics store, you could build it for like $20 & it will last forever. Also attaching a pic of the unit I built. There are 2 OE styles for these cars - one round, the other rectangular. They mount under the dash on the firewall between the fuse block & brake pedal, so unless you're looking for 'concours correct', no one is going to see it.

R.R.

How to Build a Delay Before Turn Off Circuit with a 555 Timer

https://www.digikey.ca/en/resources/...-time-constant
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