My lights-on reminder buzzer design

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Aug 14, 2014
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Madison, WI
My Car
1971 Mach 1 w/408C stroker
After reading some posts about the relay bracket above the glove box and realizing that there was an option to turn the buzzer on when the lights are left on I thought I needed to have it. I am always worried about leaving those lights on during the day so I need some type of warning. I learned that there is a relay that activates the same key-in buzzer when the door is open and the lights are on. I had deactivated the key-in circuit because I thought it was very annoying. However, I really wanted this lights-on buzzer. So I took the task of designing my own based on the existing wiring. I wanted the buzzer to turn on when the driver's door open, when the lights are on but also when the key is out of the ignition. I thought this will narrow it down to those situations where I need to be reminded to turn the lights off. Off course, you can make this whole thing a lot easier if you take away the key-out condition. In this condition it will mimic the factory design so you just need to add a relay. But being me, that I like to tinker with stuff, I wanted to stick to my key-out condition. That said, I designed a circuit that includes a transistor and a relay. I was already playing with transistors to turn some lights on when connecting my Accusump so I already had the parts for that . For relay I used the Directed Electronics 8616 Relay, which is a neat small relay that I have used in many other applications in my car. All the wires needed for this project were available at the relay bar above the glove box, so wiring was minimal.

Post for reference on circuits above glove box:
Directed Electronics 8616 Relay:
Transistor (IRF3205 MOSFET 110amp):
My old buzzer was shorted so I needed a new buzzer so I found a Cole Hersee 4099:

Here is the circuit schematic. Wires 159 and 158 are at the OEM buzzer connector. Since I disconnected the OEM key-in buzzer I used two spade connectors to tap on these. Wire 14 is part of the wiring to the lights reminder relay. I don't have this relay but there is an open connector with this brown wire that you can access with a male bullet conector. This whole thing required no cutting of the OEM wiring.
20230205_223928.jpg**read update below. This may not work w/o adding a diode between buzzer terminals!

My transistor and resistor system all bundled inside a wire sleeve. I used a 500 ohm resistor. The resistor and wires were soldered directly to the component legs.

and the relay all connected. Eventually the relay and transistor assembly were secured with cable ties.

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Update on this gadget. While it worked for a little, the mosfet "broke" and stayed on. I replaced it and worked some but then changed to continuous "on" state. I think the problem is that flyback from the buzzer may be damaging the mosfet so I may have to add a diode accross the terminals. I have plenty of diodes and few mosfets so I may have to try once more. If anyone is an expert in electronics please let me know your feedback to see if this makes sense. Otherwise I will use another relay.

I know...... why go through all this trouble for such a silly fanciness? but again, this is an easy project to tinker and learn with :D
So @TheRktmn had a very good suggestion that involved connecting a piezo buzzer between ACC and lights - this is an easy solution. My initial idea was that the buzzer starts when the key is removed and the door is open, so stubborn me continued this route. I ended up adding a diode between the terminals of the buzzer to eliminate flyback and replacing the mosfet with a relay. Now it works like a charm. I tried many times and still works. It's kind of ugly to have two relays to get this to work but it functions as I envisioned, plus these relays are very small.
This has been a good learning experience in electronics. I am pretty sure that the problem with the mosfet was the buzzer flyback, which sends a reversed voltage once it is turned off. The buzzer I am using is electromagnetic instead of a piezo. I like the old school sound that it makes.
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