Early 72' & 73' Seat Belt Warning Lamp, How does it work? Options

[Superhawk996]

Ever wondered how the seat belt warning circuit works in early 72’ & 73’s?

Lets start with the description of operation in the owners manual:

“An audio-visual warning system will remind the driver and the passenger to put on their safety belts. The buzzer will sound and the fasten seat belt sign on the dashboard will light up when the ignition is on and the parking brake is released. The warning system will also be activated if the passenger is not wearing the seat belt.”

The entire “logic” to make all this happen is contained in the seat belt warning light. The whole thing is on a small PCB (see picture below). There really is no smarts or logic in the modern sense. It is just a very simple transistor based circuit that reads switch inputs.






Note: one of the original diodes in this picture has been swapped for a 1N4001.

The vehicle side electrical connector slides onto the four PCB traces to make the connection to the circuit board.

The schematic is contained in the Haynes Manual for the 73’ model year. In my copy of Haynes the 72’ schematic is there but isn’t printed as clearly not sure if this is the case with all printings. Item 66 is the buzzer. The buzzer (which is really just a solenoid) is self actuated meaning that when 12v and ground is supplied to it, the solenoid repeatedly makes & breaks the connection that energizes the solenoid, causing it to "buzz". The buzzer has 12V to it at all times and the ground path is supplied though the transistor on the PCB.




When the Fasten Seat Belt lamp is on, the seat belt buzzer will also be on. From here on out, I’m going to neglect to mention the buzzer. But remember, if the Fasten Seat Belt light is on, the buzzer would be on also and is sourcing current through the Fasten Seat Belt PCB circuit.

NOTE: Special thanks to Montreal914 for donating this part for me to document and modify

[Superhawk996]

What isn’t shown in the schematic - the component details:

1 BC338 transistor (NPN type) – obsolete but data sheet is on Mouser, Digikey, etc. Note: Currently working to identify a suitable substitute I can swap and test. 2N3705 looks like a good substitute. The catch here is that the transistor needs to be TO-92 size to fit inside the very small Fasten Seat Belt housing. It also needs to have a rather large collector current to support the incandescent lamp current (80-100 mA) + the buzzer solenoid current (TBD). The BC 338 data sheet lists Ic as 800mA, 625 mW power capacity.

2 resistors – 1K ohms (brown, black, red)
1 resistor – 560 ohms (green, blue, brown)

3 diodes – exact type unknown marked ITT. Blue glass case with 2 cathode bands but no numerical ID numbers. Measures as 0.6v forward current drop so its a silicon diode. Suitable replacement 1N4001 silicon diode tried and tested OK.

Bulb incandescent Osram 12v, 1.2W. (grain of rice style bulb - actual current draw 90mA). Future project – replace with LED.