Electronic Horn Relay for a Bugeye Sprite
Steve
Maas
Long Beach, California, USA
January, 2007
Bugeye Sprites, as well as many other older cars, have an inherent problem with the horn. Unless a horn relay is used, the surprisingly high current required by the horn tends to fry any electrical contact in the horn circuit. In Sprites, the sliding "brush" that completes the circuit in the steering wheel is particularly vulnerable, as is the horn button itself. A simple cure is to install a horn relay. A mechanical relay has a delay of a tenth of a second or so, which is no problem for a long blast, but a short toot can get lost. Being an electrogeek, my solution was to make an electronic relay, of sorts; more correctly, an electronic switch.
The Circuit
The circuit is really pretty simple. It consists of a PNP transistor in a common-collector configuration and a couple extra components to prevent electrical transients from frying the transistor. The two wires from the wiring harness that normally connect to the horn now connect to the circuit, and two wires from the circuit connect to the horn. Simple.
The figure below shows the circuit. The MJE2955 power transistor is best for this application, but any device that can handle at least 10 amps, with a breakdown voltage of 40V or more, should be OK. The protective diode must handle about an amp and have a breakdown voltage of a few hundred volts, to be safe; I used a 1N4006. A 2N2955 will work instead of the MJE2955; that's the same chip in an older TO3 package. The package of the TO3 is the collector contact, which goes to ground, so it would not need be isolated. Same story for the tab on the MJE2955. To see a larger picture, click on the thumbnail.
Note that this is correct for a car with a negative-ground electrical system. A car with a positive ground would require an NPN power transistor such as the MJE3055 and reversed diode. The "+12V" point in the above figure then would become "-12V" of course. No other modifications would be necessary.
Instead of putting all the components on a circuit board and mounting it in a box, I simply soldered everything together and potted it in epoxy. The blob is mounted on a strip of brass shim stock, which provides the collector ground. Finally, the whole thing is mounted on the back of the aftermarket horn I use (because it's louder than the stock horn).
The tabs from the harness must be connected correctly. Once they are disconnected from the horn, the "hot" (12 volt) one can be identified with a voltmeter. An incorrect connection probably would fry the transistor and blow a fuse as well.
This circuit really should do the job reliably. If a transistor is blown, the horn will go on and stay on; it almost certainly will not blow the fuse. Then, all that's necessary is to pull over, smile sheepishly at the young gang types in the car ahead, pop the hood, and reconnect the wires from the harness to the horn. Then, the conventional horn circuit is back in place.
The circuit reduces the horn-button current from about 3 amps average (~6 amps peak!) to about 0.15 amps average. This prevents burning of the horn-button and brush contacts, so the horn might actually work when needed. Always useful.
Disclaimer
I don't like saying this, but I suppose it's necessary, now that the lawyers have taken over American society. If you choose to do this, or something similar, but don't know enough about automobiles or electronics to be comfortable with it, get some help. In any case, I'm not forcing anyone to make this modification, so if you choose to attempt it, you take full responsibility for the results. This is just a report on my experience with these modifications. It is not intended to be a set of instructions for duplicating my work or a recommendation to do it. You're on your own.