This circuit is an RPM counter. The main aim of this circuit is to count no.of rotations made in a minute by a simple wheel. This circuit is divided into 3 parts:
1) Sensing of the rotations
2) generating the pulse signals
3) Display on the LCD
The circuit makes use of simple Monstable multivibrator operation. It uses IC 555, capacitors, resistors and a battery. When the trigger input is low, then the output remains high. When the trigger is high, output remains low. The triggering is changed in this circuit by the help of a photo transistor. Let us consider a holed rotor all along its periphery. When the light falls on it, it acts as a short circuit and triggering is made low as it is grounded. Then the output is high. When the light is blocked by opaque part of rotor, the trigger is voltage divided from +VCC. Thus output remains low. Whenever hole comes, light falls on transistor and based upon this the output pulses are generated.
Again to make 5 pulses for 5 holes as one pulse, we use another similar multivibrator circuit by changing the width of the pulse by changing the resistance of second 555 multivibrator.
The display is done by Seven Segment display. To know about seven segment display, see this link.
Now we get number of cycles per second.We can calibrate it for Rotations Per Minute (RPM).Multiply the output from multivibrator-2 by a factor 60 by placing a multiplier (using an op-amp).After this give it to Seven Segment Display.
Simulated circuit is below:
We can compare output of one multivibrator to output of the other.
Testing in myDAQ:
This circuit counts only single digit RPM. The same circuit can be further extended to count more RPM by making manipulations in seven segment display ( by increasing the number of seven segment displays) to know about how to increase the number of digits from one to two and more etc see this link .