A stroboscope & frequency counter design

V1.00  10-Dec-00

1. Introduction

To measure the speed of rotation of a motor, or any other part of teh robot, it is useful to have a stroboscope. This allows a non-contact method to measure the rotational speed, simply by drawing a chalk mark on the item to be measured. The area is then lit by a flashing light, whose frequency is adjusted upwards until the mark appears stationary. The rotational speed is then equal to the frequency of the flashing, which can be read off the display of the stroboscope.

The design here uses a voltage to frequency converter driving a high-brightness LED to generate the flashing light, and a single-chip frequency counter IC from Intersil to drive a 4-digit 7-segment display to display the frequency. The frequency goven is in Hz, to convert this into RPM, use:

rpm = frequency * 60

2. A strobe circuit

2.1. Circuit diagram

2.2. Circuit description

The AD654 is used to generate the strobe frequency. This converts a voltage (from teh potentiometer) to a logic level frequency. The transistor then drives the high-brightness LED. The circuit values for this part is directly from the AD654 datasheet. The trimmer resistor is to set the maximum range of the frequency. We need a maximum of around 20,000rpm, which is 333Hz. The capacitor value sets this range, which is fine tuned with the trimmer.

Parts datasheets:

Part Datasheet URL
ICM7224 http://www.intersil.com/data/FN/FN3/FN3168/FN3168.pdf
LM358 http://www.intersil.com/data/FN/FN1/FN1019/FN1019.pdf
AD654 http://www.analog.com/pdf/AD654_b.pdf

3. The frequency counter / tachometer section

3.1. Frequency counter on a meter

Some DMMs also have a frequency counter mode on them. If you need to buy a DMM, get one with a frequency counter. Note however that some of them have a very restrictive frequency range, sometimes only starting at several kHz. If you also need to measure high DC currents, the clamp meter available from Maplin is very good.

3.2. Tachometer module

Maplin also sell a tachometer section whoch can be used to measure the frequency of the flashing LED section of the stroboscope. Using this you can do away with the ICM7224 section. Maplin order code VE01B.


3. Links