AC Powered Engine Timing Light




About: I miss the days when magazines like Popular Mechanics had all sorts of DIY projects for making and repairing just about everything. I am enjoying posting things I have learned and done since I got my first ...

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.



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    10 Discussions

    Phil BJunever escobar

    Reply 2 years ago

    You will need:

    1 xenon flash tube
    2 capacitors rated at 500 volts and 30 microfarads
    1 300 Ohm 20 Watt resistor
    2 diodes
    1 trigger switch
    1 focussing lens
    Ignition wire
    Electrical cord, trigger switch, and a wooden case

    Forty years ago when I made this, I got parts at an electronic parts store and I used the model number from Sears timing lights to order a lenses a replacement part. I got copper core ignition wire at an automobile dealership. Now I wonder if you could not adapt the flash circuit from a disposable camera. You may be able to salvage some of these parts from old electronic devices.

    I did not have access to any old television sets at the time, and have rarely had access to any since. But, it is a good idea. I remember doing some research on the matter at the time and there was a difference between xenon tubes for party strobe lights and xenon tubes for engine timing lights. I no longer remember what it was, but I passed on the xenon tubes at Radio Shack because they did not meet the appropriate specs. Thanks for your comment.


    9 years ago on Introduction

    Sweet, a blast from the past!

    I do remember my father had one of this timing lamps but if I recall right, the light was redish meaning it was some kind of neon, isn't?

    Anyway, congrats for such a nice instructable.

    I gave it 5!

    1 reply
    Phil Bstormende

    Reply 9 years ago on Introduction

    Your father's timing light was almost certainly a neon timing light.  They were  inexpensive and required no power other than the current for the spark plug.  They also gave very little light and worked better used inside a dark building or after the sun had gone down.  I had one for a while and taped it to a stick so I could get its light closer to the damper pulley with the timing marks, but without getting my hands into the engine's belts and fan.  In the late 1960's a neon timing light cost about $5 US.  The AC powered light I copied for the light shown in this Instructable cost $20 US.  A nice DC powered timing light powered from the car battery was about $35 US.  The legally proscribed minimum wage was about $2.75 US and gasoline was about 34.9 cents a gallon.  You can easily see why not everyone had a powered timing light, AC or DC.  Thank you for your comment and your very favorable rating.


    9 years ago on Introduction

    thanks for you and keep it on, do not let any thing to stop you becoase its been sold for more than 100$

    1 reply
    Phil Bmortaz

    Reply 9 years ago on Introduction

    Mortaz, Thank you for your comment. If the line voltage where you live is greater than 120 volts, say 240 volts, you would need to use electronic parts capable of using higher voltages without failing. Instead of parts rated at 300 volts, you would probably need parts rated at 500 to 600 volts.

    Phil B

    9 years ago on Introduction

    Thanks. I have posted a bunch of stuff I did over the past 40 years. Take a glance at some of my other Instructables. Not so many people use timing lights anymore because of changes in auto design. Even fewer have heard of an AC powered timing light. But, there still are times when one comes in very handy, and an AC powered light is pretty easy.