Electronics Mini Projects using OPAMP: Automatic Lamp ON project
Electronics Mini Projects using OPAMP:
Project Name: Automatic Predefined Time Lamp Turn ON Project
Imagine that in your house you want to go to staircase in the evening. You want the light has to glow during the time you are in the steps. Once you reached the staircase the light is not required. It has to be turned off. You can achieve this by two-way switch which is very old concept. You can use the below mentioned circuit to achieve for that purpose.
Another real life example:
 Have you absorbed in some hotels whenever you open the floor door the additional lights will glow for some time ( say 3 to 5 mins) in the corridor.
 In some pubs after opening the main door, the Lamps will glow for few minutes...
So we can name the project as follows too;
Automatic Staircase Lamp ON Project
Automatic Corridor Light turn ON Project Based on Opamp
Change V+ pin no 8 into 7 and for output use pin-6 instead of 1, if you are using 8 pin PDIP package op amp IC.
The Pin diagram of IC741 is given below)
- This is a simple project which consists of cheap timer circuit using OP AMP IC741.
- The timer's time period is determined by the RC constant.
- Thus the Light glow time duration can be changed by changing the values of R and C.
- Whenever the Push button is pressed, the lamp will glow. After the time period decided by RC values, the light will switch off automatically.
- When the push button is pressed, the output of the IC at pin 6 becomes low. It will energizing the relay resistance R.
- The Normally Open (N/O) point of the relay is connected across the switch S1. As the relay gets energised, the point is closed.
- So even on releasing the push button S1, the timer stays on. ie the lamp will glow.
- Till the time, the timer stays ON the current is supplied to the lamp.
- After the passage of the present time determined by constant RC, the relay deenergises and the lamp goes off.
- As no current flows from the battery during the OFF period, battery life is increased.
Parts required to build the circuit:
Timer - IC741
Resistance - 2Meg, 820K Ohm
Capacitor - 10uF
Relay ( Voltage should be 9V and internal resistance should be more than 200 ohms)
Push button type switch.
Circuit Components in details:
The pin diagram of the opamp IC
It is recommended to get refresh about the opamp basics and specification parameters before starting to work on the breadboard.
Operational Amplifier Parameters:
Input Offset Voltage:
It is the error voltage required to make null the quiescent output voltage of the operational amplifier. ie a voltage that must be applied between the two input terminals of a opamp to null the output. IC 741 has an input offset voltage of 5mV
Input Offset Current:
It is the difference between the two input current of the operational amplifier. Opamp fabrication uses manolithic integrated circuit construction.
Input Bias current:
It is the average of the current that flow into the inverting and non inverting input terminals of the operational amplifier.
Common Mode Rejection Ratio:
The common mode rejection ratio is defined as the ratio of the differential voltage gain Ad to the common mode voltage gain Acm
It is defined as the maximum rate of change in output voltage per unit of time and is expressed in volts per microseconds.
Slew rate indicates how fast the op-amp output can change in response to the input frequency. Opamp IC741 has a slew rate of 0.5V/us.
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