COMBINED COMPONENT CIRCUITS
The Automatic Night Light Circuit
Background Knowledge:
How to draw a Schematic
How to read and use a Multimeter
Current Flow in a Series Circuit and Parallel Circuit
What you'll need:
1 - 9 Volt Battery
1 - 9 Volt Battery Harness
1 - Breadboard
1 - 1N4007 Rectifier Diode
1 - 470-Ohm 1/4 Watt THR
1 - 22 K-Ohm 1/4 Watt THR
1 - 100 K-Ohm Potentiometer
1 - 1 M-Ohm LDR (Dark)
1 - 3904 NPN Transistor
1 - Red 5mm LED
What to Expect:
Attach your power supply.
Turn the knob on the pot one direction until the LEDs are barely off.
Now, cover the LDR completely. The LEDs will turn on.
If Move to a semi-lit area. The LEDs will dim as you move into the light. Adjust the pot so the LEDs are again barely off. Any reduction in light will now turn the LEDs on. This is your automatic night light.
How it Works:
The potentiometer adjusts the amount of voltage shared by the 22,000-ohm resistor and the LDR.
In light, the LDR has low resistance, allowing all of the voltage to flow through to the ground. Because of this, the base of Q1 gets no voltage and the valve from C to E stays open.
The resistance in the LDR increases as it gets darker limiting the current that will flow through the LDR providing more voltage to the base of the transistor, pushing the valve open. As the current flows through the transistor, the LEDs turn on.
If POT1 is set to a low resistance, more voltage gets through. The more voltage that gets through the potentiometer, the easier Q1 turns on because the LDR cannot dump all of the voltage.
Steps
Set the potentiometer so the resistance between legs “A” and “C” is close to 50-kilo-ohm resistance.
Now set your circuit under a fairly good light.
Now measure and record the voltage over the potentiometer.
Now cover the LDR with a heavy dark pen cap.
Measure and record the voltage over the potentiometer again.
The pot uses some voltage because it is set to 50,000 ohms here. The LDR used a small amount of the voltage in the light because it has a small resistance where the current would then flow directly through the LDR to ground. When it is in the dark, the LDR has a great amount of resistance which blocks significant current flow in that direction. The base of the transistor reacts to the voltage available at that point where the LDR and potentiometer connect. It becomes obvious which situation provides more voltage to the transistor base.
Set the potentiometer so the resistance between legs “A” and “C” center now has zero ohms.
Now set your circuit under a fairly good light.
Measure and record the voltage over the potentiometer.
Now cover the LDR with a heavy dark pen cap.
Measure and record the voltage over the potentiometer again.
When you set the potentiometer to zero ohms, you no longer have two loads in series to create a voltage drop. The potentiometer uses none of the voltage, so the base of the transistor is exposed to nearly full voltage whether the LDR is in the light or the dark.
Set the potentiometer so the resistance between legs “A” and “C” center now has 100 kilo-ohms.
Now set your circuit under a fairly good light.
Measure and record the voltage over the potentiometer.
Now cover the LDR with a heavy dark pen cap.
Measure and record the voltage over the potentiometer again.
The potentiometer is adjusted to twice as much resistance as before, so it will use twice as much of the voltage available. The LDR must be set into nearly complete darkness to create the voltage drop needed to allow current to flow into the base leg of the transistor.
Turn the circuit on and adjust the Potentiometer so that the Night Light just barely turns off while in the light.
This should allow the Night Light to be at its most sensitive so that as soon as the room loses any significant light, the LED turns on.
Questions:
Why do you need to use a Potentiometer in this circuit?
Once you tune the potentiometer to resistance that allows this nightlight circuit to be at its most sensitive, what could you do?
Did the LED output change at all?
How does the circuit depend on the amount of voltage available?
LED Blinker Circuit
Background Knowledge:
How to draw a Schematic
Resistors, Capacitors, Transistors, and LEDs
Current Flow in a Series Circuit and Parallel Circuit
What you'll need:
1 - 9 Volt Battery
1 - 9 Volt Battery Harness
1 - Breadboard
2 - 470-Ohm 1/4 Watt THR
2 - 100 K-Ohm 1/4 Watt THR
2 - 10 uF 35V Radial Capacitor
2 - 3904 NPN Transistor
2 - Green 5mm LED
What to Expect:
When you power this circuit, the LEDs will alternate flashing lights.
How it Works:
The 3904 NPN Transistors are open circuits that do not allow current to flow until the base leg has voltage and current on it.
When power is supplied a small amount of voltage will prime the base leg of the opposite transistor of each 100K resistor. It also creates a small voltage potential on the negative pole of the capacitor on its side.
When the first transistor is closed and allowing current to flow from the collector to the emitter. For as long as the voltage potential on the positive side of the capacitor is more than the negative side while the capacitor is discharging, that side's LED remains on.
When the capacitor drains, the transistor on the same side will open and the opposite transistor will close restarting this cycle on the other side of the circuit.
Questions:
How is current flowing in this circuit?
What can you do to this circuit if it does not flash? Why would it do this in the first place?
For more advanced circuits, please continue on to our Logic Gates and Integrated Circuits pages. These pages will explore digital electronics in great detail with more projects to complete.