AC Powered Engine Timing Light

Back in the 1970s I wanted a xenon timing light to replace the nearly useless neon timing light I had. I borrowed a friend's AC powered timing light to use. While I had it, I opened it and made a diagram of the circuit. Then I went to an electronics supply store and got most of the parts. I got the lens and the xenon flash tube from Sears. To do that, I took the model number from a new unit on the shelf and went to their repair parts store. In a few minutes I had the part numbers I needed and ordered them. Today you can use an Internet search to find replacement parts for xenon timing lights. I decided on an AC circuit because the circuit is simpler and because I can use the light on machines with a magneto ignition where there is likely no battery.

Once we had a yard full of gopher mounds. The soil was clay. When I hit one of those with my mower, it often sheared the flywheel key. I eventually put timing marks on that mower so I could check it with this timing light before tearing the motor apart and learning it was not running for some other reason.

Be careful: this circuit makes use of high voltage. Before handling internal parts use a screwdriver with a plastic insulated handle to remove the charges from the capacitor by shorting out the case or ground terminal to the "+" terminals of the capacitor. Do this a couple of times to be certain all charges are removed.

The paint was from an aerosol can of touch up paint for a 1963 Chevrolet.

Step 1: A Wooden Case

I made the case from plywood. I began by cutting a pistol grip base from two pieces of 3/4 inch plywood. I made recesses for the trigger switch, the AC power cord, and for the copper core spark plug wire. There is a hidden pivot pin in the 1/4 inch plywood trigger.

Next I cut and glued the right side panel to the side of the base. Then I framed the light with a back, top, and front. The front has a hole for the lens.

You can also see wedges to hold the wires in place in the pistol handle.

Step 2: Internal Placement of the Parts

This photo shows the placement of the electronic parts. I also traced the wiring diagram into the photo with different colored lines to differentiate the conductors and to make the circuit easy to follow. The dotted portions simply trace the circuit path when the conductor is hidden behind another conductor. The azure blue and lime green lines at the left of the photo go to the flash tube terminals. The lime green conductor actually connects at the ground or case terminal on the multi-section capacitor, although it appears to connect to the diode lead, which would work, too. The deep purple lines at the left of the photo show one of the AC power lines and the switch activated by the wooden trigger. The maroon line is the other AC power line. It passes through a 300 ohm 20 watt resistor. Then it splits for two diodes. Note that on one the anode comes first, while on the other it is the cathode. A multi-section electrolytic capacitor was used, but two individual capacitors rated at 30 microfarads and 500 volts each could also be used. The triangle and the "D" indicate separate internal terminals on the multi-section capacitor. The case of the capacitor is indicated by the maroon ground symbol. See the schematic in the next step.

Step 3: The Actual Schematic

I have tried to be accurate in drawing the schematic used in this timing light, but included the photo with colored lines, too, so that you can cross check what I have done. Notice the red "D" and triangle to indicate the capacitor terminals. The ground symbol indicates the capacitor case. The diodes should be able to handle 500 volts. (Note: This graphic erroneously shows a 120 volt DC power source. It does use an AC source.)

Step 4: A Voltage Doubler

This is the schematic of a standard voltage doubler circuit. It would work, too.

Step 5: The Switch

In addition to the plywood trigger for the switch, I used an ordinary safety pin to provide a spring. I also used two tabs of aluminum. I bent the pointed end of the sharp point over at a right angle and stuck it in its wood backing. A small screw through the spring loop on the safety pin serves as an axis for the safety pin spring. You can also see the upholstery foam I used to mount the flash tube and protect it from bumps and shocks.

Step 6: Another View of the Switch

Here you can see my thumb pressing the wooden trigger. It elevates one of the aluminum tabs so that it touches the other mounted above it and completes the circuit.

Step 7: Flash Tube and Lens

This shows the "U" shaped xenon flash tube riding in a slit I made in the upholstery foam. For a trigger circuit I simply wrapped some of the bared spark plug wire around the flash tube. The hole for the lens is drilled to fit somewhat tightly. I used auto body adhesive around its edges to hold it in place. I also lined the flash tube cavity with white card stock to reflect as much light as possible toward the lens.

Step 8: Connection to the Engine's Spark Plug Terminal and Wire

There are inductive pickup coil circuits you can find on the Internet. One of these could be integrated into this timing light. I chose to use a direct solid connection. I have experimented with springy coiled wire that was supposed to fit over the end of the plug and into the end of the spark plug wire. Those were not completely satisfactory. Finally, I put a piece of 1/4 inch rod into a drill and held the end near to a spinning grind stone. I turned a profile similar to what you find on top of a spark plug. I fastened a terminal from a kit that allows you to make up your own spark plug wires. It is not fancy, but it works.

I have not used this timing light for many years. My current auto does not even have spark plug wires, but a buss under the valve rocker arm cover. I did connect it to the distributor on my wife's car and the timing light still works 36 years after I made it. I really expected the capacitor would have failed by now, but not so.