Steve K
Flashlight Enthusiast
Somehow, a conversation with a friend led to the Belt Beacon, a bicycle strobe light from the mid 70's. I used one back then, and it was a big improvement over the other humble lights that were available for bikes. It used actual electronics!! It could run from a 9v battery for hours, and was much more visible than other lights (although that seems unlikely in retrospect). In any case, it was the best available for quite a few years.
Naturally, I had to go dig through my parts box and see if I ever got rid of it. Somewhat to my shame, I still had it. Here are a few shots of it....
so what was in this light??
Well, you had to pop off the amber lens and remove two screws to remove the plate that the bulb was mounted to...
amazingly, the screws that hold the light together are also parts of the circuit.. they connect two wires from the board to the contacts for the bulb. The wires from the board are fitted into the threaded sockets that the screws go into. This is certainly a case of thinking outside of the box!
The circuit itself was very simple. Two transistors, three resistors, and one capacitor.
The board itself was single-sided, with no jumpers on the top of the board. This is harder to do than you might think.
I reverse engineered the circuit, which is much easier to do than with modern circuits. The values and part numbers are all clearly marked, and the traces are all visible. Piece of cake! The only complication is that the transistors are no longer being made, and their manufacturers, Motorola and National Semiconductor, are no longer in the business of making transistors. I did have to probe the transistors with a meter to figure out if they were npn or pnp (answer: one of each).
Here's the schematic....
I even ran a simulation using LT Spice to see if my schematic made sense. It didn't work that well with generic transistors, but substituting common 2N3904 and 2N3906 transistors caused the simulation to work fine. The voltages shown below did help me get a better feel for how the circuit works, and just how clever it is.
The bulb flashes roughly once a second.
the blue trace is the voltage at Q2's base. The green trace is Q1's collector, the low side of the bulb.
Okay.. I'll admit.. this may be of interest to just a few folks, but I felt this was definitely the place to share.
enjoy!
Naturally, I had to go dig through my parts box and see if I ever got rid of it. Somewhat to my shame, I still had it. Here are a few shots of it....
so what was in this light??
Well, you had to pop off the amber lens and remove two screws to remove the plate that the bulb was mounted to...
amazingly, the screws that hold the light together are also parts of the circuit.. they connect two wires from the board to the contacts for the bulb. The wires from the board are fitted into the threaded sockets that the screws go into. This is certainly a case of thinking outside of the box!
The circuit itself was very simple. Two transistors, three resistors, and one capacitor.
The board itself was single-sided, with no jumpers on the top of the board. This is harder to do than you might think.
I reverse engineered the circuit, which is much easier to do than with modern circuits. The values and part numbers are all clearly marked, and the traces are all visible. Piece of cake! The only complication is that the transistors are no longer being made, and their manufacturers, Motorola and National Semiconductor, are no longer in the business of making transistors. I did have to probe the transistors with a meter to figure out if they were npn or pnp (answer: one of each).
Here's the schematic....
I even ran a simulation using LT Spice to see if my schematic made sense. It didn't work that well with generic transistors, but substituting common 2N3904 and 2N3906 transistors caused the simulation to work fine. The voltages shown below did help me get a better feel for how the circuit works, and just how clever it is.
The bulb flashes roughly once a second.
the blue trace is the voltage at Q2's base. The green trace is Q1's collector, the low side of the bulb.
Okay.. I'll admit.. this may be of interest to just a few folks, but I felt this was definitely the place to share.
enjoy!