Wireless Horn for 1994 Ford Tempo

The horn of my 94 Ford Tempo has been not working for a while. Based on my understanding, the parts involved in the horn circuit are: the battery, the horn switch, the fuse, the horn relay, the horns and the wiring. I did some trouble-shooting. The battery is good (or my car won’t start). The fuse is good. According to the fuses identification chart below (which was found in the glove compartment), the horns share the fuse with the cigarette lighter.

In fact, I pulled out all fuses from the fuse panel and tested them. So, the remaining suspects are the horns, the horn relay and the wires. I wanted to run two wires from the battery to test the horn as suggested in some posts found on the internet, but changed my mind later because getting access to the horns is a big job for me. See the post below:

I can’t locate the horn relay either. I guess it shares the same compartment where the horns is. Then, another idea came up - to run an alternative horn circuit and mount a new horn somewhere under the hood. The circuit looks like: The battery (+) à in-line fuseà horn switchà horn relay àhorn(s) à chassis. It looks easier because I don’t need to get access to the stock horns.

Now the big challenge is to run a pair of wire from the horn switch, through the fire wall to the horn somewhere under the hood. I looked around under the hood and puzzled. I needed to drill a hole through the fire wall. Forget it, it is too risky. It could cause a very expensive damage if, say, a hose or wire harness got drilled. I changed the plan a little bit. I decided to go for wireless. It eliminates the big headache for drilling the firewall.

I looked around the house to see what wireless thing I can use. I got a remote control toy car, a wireless door bell, and a retired cordless phone. I chose the wireless door bell because the mod is easier.

Wireless Door Bell Mod

Open the door bell’s enclosure and get access to the PCB assembly. While operating the transmitter ON and OFF repeatedly,  probe the receiver’s PCB with a scope (or a digital voltmeter) to find a point where the logic level toggles with the operation of the transmitter. That point will be used as a receiver’s output for driving the relay circuit. Found it. See the picture below.

 I also decided to build an electronic horn using parts found in my junk box. It is just for fun and hardly justified from the point of view of saving money.

10W RMS Electronic Horn circuit

The horn circuit consists of a multi-vibrator, a buffer, a power MOSFET and a midrange speaker. The frequency can be varied by R5. I adjusted it until the frequency is around 600Hz. The transistor TR1, TR2 and TR3 can be any small signal NPN transistor. TR4 can be any N-channel power MOSFET that can handle 5A, 50V. The diode is a 1A 50V silicon diode. The speaker can be any 4 - 8Ω midrange speaker that can handle 15W RMS. The transformer is a 110V/12V 12VA power transformer. The 110V winding has no connection. The transformer is used as an inductor. It store energy while Q4 is ON and release the energy to the speaker when Q4 is OFF.

Relay Driver circuit

The horn circuit and the relay driver circuit were assembled with a prototyping board and put into the battery compartment of the door bell.

At 12V, it draws 0.8A. The horn is not very loud because the power supply is only 12V which is not high enough to drive an 8 Ω speaker as a car horn. Several alternatives have been evaluated to increase the output power.

1.                  Using a piezo buzzer will get a much louder sound at a smaller current but piezo buzzer doesn’t sound like a car horn. It resonates at 3 – 5kHz whereas the car horn operates at around 600Hz.

2.                  Using a 4Ω speaker so that higher current can be obtained with the same voltage supplied.

3.                  Increase the supply voltage (DC-DC conversion) so that higher current can be obtained with the same speaker impedance.

4.                  Using an h-bridge driving circuit so that the amplitude of the speaker cone is double.

I will try (3) the DC-DC conversion and (4) the h-bridge driving circuit later. I will also hunt for a 4Ω mid-range speaker. You may like to use a commercial electromagnet horn instead of building the electronic horn. Just make sure that the relay and the wire should be of appropriate rating for the horn you use.

Note: Tried (3) and (4), it didn't improve much (May 3, 2008).

Installation

The installation is simple. Find a spot under the hood to mount the wireless horn and the speaker.

Run a wire to the battery (+) terminal via an in-line fuse. Run another wire to attach to the car chassis.

Stick the transmitter onto the steering wheel and test it.

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End of the project

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