Introduction:-
This project is a simple frequency indicator. It displays number of pulses passed for the set time interval "T". If this time interval is of 1 second then we will get the frequency, whose value will be indicated by the number of lit led's. These led's are connected to the output of 8 bit shift register ( IC 74595) , this limits the maximum frequency that can be indicated by the led bar graph to 8 Hz. To measure higher frequencies we can decrease the time interval or cascade two or more shift registers to display more number of counts. The circuit diagram of the project is given below:-
Understanding this circuit requires basic knowledge about shift registers and edge detector circuits. I would recommend the reader to watch the below given videos for a clear understanding. It benefited me in making many other circuits.
For shift register:- https://youtu.be/6fVbJbNPrEU
Working (from left to right):-
1. The light dependent resistor (LDR) is used as a light sensor. When light falls on it it's resistance drops which indirectly drops the voltage accross it.
2. This drop in voltage is detected by the negative edge detector circuit which outputs a positive trigger pulse. This positive trigger pulse also acts as a serial clock pulse for shift register. For triggering the 555 ,The positive trigger pulse is converted into negative trigger pulse by transistor BC547.
3. The 555 monostable multivibrator (MMV) circuit detects this negative trigger pulse and pulls the output high for time ( T ) given by formula 1.11RC. In this circuit T=1 sec. This output is given to serial input data pin and serial clear pin of shift register.
4. For the time, the output of 555 is high (which is 1 sec). The data '1' is shifted each time a flash of light is detected by the LDR. After 1 sec when the output of 555 turns 'low'. A negative edge detector gives a positive trigger pulse to the register clock pin of shift register. This latches the 8 bit data of storage register to the output pins ( Q0 to Q7 ) of shift register.
5. If the flash rate of light is 2 Hz ,then data in storage resistor will be "11000000". Which will be latched to the output after 1 sec.
6. Only non-retriggerable monostable multivibrator (MMV) will work for this pulse counting circuit as the MMV cannot be retriggered for set time interval. The 555 MMV is non-retriggerable type. watch this video for more info:- https://youtu.be/tyD5Ym1xKn0
To see working demonstration of the project visit https://youtu.be/HVh95YBNv2M
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