Transistorized Ignition Circuits
The plans for the radial engine called for a set of mechanical breakers and a CDI ignition system. Being a "modern" guy, I thought I'd be smart and convert the distributor and circuit to utilize a hall effect IC to activate the circuit. A lot of guys are using hall-effect with great success, but I was not one of them. After much research and perhaps a dozen blow Hall IC's, I gave up and went with mechanical breakers to drive this circuit.
My error: I placed the hall IC inside the distributor, where it was subject to high voltages induced via arcing and other wierdness. Even after optically isolating the chip, it would still blow... so don't design your system with the Hall chip inside the distributor unless you enjoy pulling your hair out and wishing your hobby was glueing popsicle sticks into little forts and such. Don't say I didn't warn you!
The circuit shown below was first published in Strictly IC. I have built and tested several of them, and the circuit works like a champ. It is designed to interface with a Hall IC chip, a device which will switch when presented with the south pole of a magnet. When the magnet is exposed to the chip, the circuit switches ON and supplies power to the ignition coil. When the magnet is removed, the coil fires and supplies spark to either a single plug, or via a distributor to several plugs.
The presence of the magnetic field over the chip corresponds to the dwell time of a traditional ignition system, while the removal of said field corresponds to the breaker points opening.
The circuit will work with mechanical breakers as well... simply remove the H1 line, and connect line H3 to the breaker. H2, of course, is grounded to the engine frame.
R1, R2 - 100 ohm 1/8 watt
I use this circuit with 12V batteries and coils with no problems... all of the currents are within the parameters of the transistors per their data sheets. The R4 value was originally 3.9 ohm; this caused T2 to sink nearly an amp, well out of limits. The 20 ohm resistor fixed that, but in anything less than a 5 watt power rating, it tends to get a bit toasty. Experimentation is always in order.
This circuit is the utmost in simplicity and power. It is currently running the radial engine with a full-sized automotive coil with great success. It is designed for 12VDC operation, and the breakers will sink ~300 mA, a bit higher than the TIM4, but it does do away with several components.
R1 - 40 ohm
5 Watt power resistor
I'm fairly certain that any high powered PNP Bipolar Silicon device will work. Look for 10 or more amps CE, as high a voltage rating as you can get, and an hFE (gain) of 60 or so.
A better circuit by far than the TIM6, but with the drawback of an engine frame being +12V rather than ground. This is really no big deal if your engine is simply a running bench display. The transistor is an NPN Motorola MJ10012 power darlington, designed for ignition systems. A base current of 300 mA saturates this transistor much better than the TIM6, and the spark is nice and hot.
This circuit has replaced the TIM-6 on my radial engine.