Circuit application:
• This is a typical industrial type circuit where a voltage input needs to be sensed to turn on a relay to
switch some higher current and/or a higher voltage.
• While we are generating the external voltage here from a power supply, it represents a voltage which
could come from anywhere like a sensor, a battery, or anything else that needs monitoring.
• Relays are designed to drive something more than an LED.
? It turn on a motor or an OVEN or a large lamp.
? If you look on the relay body it says the switch can handle 10A at 250VAC
• The opamp or the transistor would not be able to do that by themselves.
• Thus the relay is needed to work with and isolate higher voltages and currents and the transistor is
needed to drive the relay.
Operation description:
1) When the external voltage V1 is equal to the reference voltage of 6V, the opamp output will go high.
2) which will turn-on the transistor
3) which in turn provides current for the relay
4) which pulls-in a switch
5) to light the LED at 12V.
Base resistor calculation:
• Find the resistance of the 12V relay coil in the datasheet. Write that here ______ ohms
• Determine the maximum collector, $I_C$, current assuming the $V_{CE} = 0$, 12V / relay resistance = ______
• Find the HFE in the datasheet for a $V_{CE} = 1.0V$ since we are operating in saturation
• Calculate the expected base current ($I_B = I_C/h_{FE}$)
• Then calculate the base resistor R4 using the calculated base current. (12V-0.7)/$I_B$ = ______
LED resistor calculation:
• Assuming 20 mA through the LED and 2V for a forward LED voltage.
(12-2)/20 mA = R7 = ______
1. What are the 2 resistors you calculated?
R4 base resistor
R7 led resistor
