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Led Strobe and Tacho Pt.2

Last month, we published the circuit of our new LED Strobe & Tachometer and showed how to build the main unit and the strobe light. This month, we describe the assembly of the optional Photo-Interruptor and IR Reflector Amplifier boards. We also describe how the unit is used.


ET’S START WITH the assembly of the Photo-Interruptor board – see Fig.11. This board is coded 04108083 and carries just the photo-interruptor itself, a 150W resistor and three PC stakes.

The assembly should take only a few minutes. Just be sure to install the photo-interruptor with the correct orientation, ie, with its diode symbol (indicated in blue on Fig.11) on the righthand side. It should be secured to the PC board using two M3 x 6mm screws and nuts before the leads are soldered.

The completed assembly is wired to a 3.5mm jack plug using 2-core shielded cable, with the shield wire used as the 0V (GND) connection (ie, it goes to the sleeve) – see Fig.6 last month (page 67). Make sure that the tip and ring connections are made correctly. The tip connection is right at the end of the plug, while the ring is the separate section just behind the tip.

The 0V or ground terminal is the main body connection. Use your multimeter to identify the jack plug terminals if you are unsure.

To test the unit, plug it into the main unit, apply power, set the unit to Trigger mode and then return to the main RPM display. If a slotted disc (or some other opaque shape) is now rotated through the photo-interruptor, a reading should appear on the display. In addition, the strobe should flash each time the light path is interrupted. If this doesn’t happen, check your connections.

In practice, this unit is intended to be used with a small slotted disk (see photo) that spins within the gap of the photo-interruptor (ie, the disk is driven by the rotating machinery). You will need to manufacture the disk to suit your application.

The completed Photo-Interruptor board can be attached to a fixed section the machine. Be sure to mount it using Nylon (not metal) spacers at the output end, to prevent shorts to the soldered joints.

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