Simple Analogue Optical Tacho
Optical Tacho Inside view
Description
The tacho uses a LDR to sense variations in light intensity to produce a square wave at the output of a comparator (LM741). This square wave is fed to a logic one shot (74LS123N) which produces a timed pulse each time it sees a falling edge from the comparator. The length of the pulse is dependent only on the sizes of C3 and VR1. Thus as the frequency increases, the output from the one shot has an average voltage value that increases proportional to rpm.
Tacho Circuit Board
Download component list Apologies for the quality of these files
Notes
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The meter is a Dick Smith Electronics (Cat No Q2045), but any small micro ammeter could be used.
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For greater accuracy, C3 could be changed to a more stable type of cap. The size of C3 may be reduced with an corresponding increase in the value of VR1.
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As can be seen from the insides view, the circuit fits very easily into a small case along with a nine volt battery.
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The tacho is positioned in the proximity of the spinning blades of a propeller. The blades momentarily reduce the light entering the LDR, which the tacho reads as a pulse. The tacho therefore senses 2 pulses per revolution. 3 or more bladed propellers will require the output to be scaled accordingly.
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The tacho can be used on any application, where a variation in light intensity can be obtained as an input. A friend of mine uses one form tuning RC electric car motors. He attaches a small disc which has 2 black and 2 white sectors.
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Fluorescent lights can cause major problems with the readings. The light from Fluorescents is of a pulsed nature. At 50 Hz, 100 pulses per second are given off. The light intensity change can be great enough for the tacho not to read all pulses from whatever you are trying to measure.
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The instructions for setting up the tacho say to set the tacho at 3000 rpm when under a fluorescent light. This is for countries that have 50 Hz mains supply. In 60 Hz countries the setting will be 3600 rpm.
Public Domain and disclaimer
This device, and any devices detailed in the "Free Stuff" pages is for public use and can be copied and distributed freely. The user accepts any responsibility for using these circuits. Don't contact me if you have a problem, you're on your own.
All other pages, circuits, photographs and documents on this website are the property of myself and are subject to international copyright laws.
Ian W. Armstrong