# Little help with this circuit pls..

#### syaf921

Joined Dec 21, 2012
18
What is the function of 741 operational amplifier in the circuit below? This is a light detector circuit.

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#### KMoffett

Joined Dec 19, 2007
2,918
Tthe 741 is used as a voltage comparator. It compares the voltages at pins 1 and 3. If the voltage at pin 2 is less than the voltage at pin 3, pin 6's voltage goes toward Vcc and turns on the Q1. If the voltage at pin 2 is more than the voltage at pin 3, pin 6's voltage goes toward ground and turns off the Q1.

Ken

#### Ron H

Joined Apr 14, 2005
7,014
Tthe 741 is used as a voltage comparator. It compares the voltages at pins 1 and 3. If the voltage at pin 2 is less than the voltage at pin 3, pin 6's voltage goes toward Vcc and turns on the Q1. If the voltage at pin 2 is more than the voltage at pin 3, pin 6's voltage goes toward ground and turns off the Q1.

Ken
Except for a couple of details. The 100k resistor is too high in value to saturate the 2N2222. If you make it low enough to guarantee saturation, the 741 output will never go low enough to guarantee that the 2N2222 will turn off reliably.
If you want the function of a comparator, then use a comparator. LM393 (dual) is usually a good choice. Be aware the LM393 has open collector outputs, so you will need pullup resistors on the outputs.

#### ScottWang

Joined Aug 23, 2012
7,306
The value of R2 is really too high, and you should reduce the value and series in a 1N4148 or add a resistor Rbe between B,E of Q1, because when the 741 output a low level is around 1.4V, that is too high and can't turn off the Q1.

#### bertus

Joined Apr 5, 2008
22,120
Hello,

In the given schematic there is also no back-emf diode across the relays coil.
Without this diode the transistor may blow when the power is removed from the relays.
The diode can be a 1N400X type with the cathode connected to the VCC and the anode to the collector of the transistor.

Bertus