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Variable Voltage Regulator – Indermediate

Building the circuit:

The parts list is as follows: A pack with all these parts can be found here

Ω      One 555 Timer

Ω      One LED (any colour!)

Ω      One 120Ω resistor

Ω      One 100KΩ potentiometer

Ω      One 5 – 12V supply or 9v battery

Ω      Two 1n4001 diode

Ω      Two 100nF capacitors

Ω      A breadboard

Ω      Some jumper cables


The circuit diagram

Varaible Voltage circuit diagram

Here is our circuit fully built up:

variable voltage power supply on breadboard  Close up of variable voltage supply

close up of power supply breadboard  Variable voltage potentiometer close up

Overview / how it works:

This is a very simple circuit to build, but a much more complex one to understand. We’ve done a few projects with 555 timers before, but never gone into any great detail about how they work. The principle of a 555 timer is quite simple, it all revolves around one key idea, the state of pin 3 (the output pin) is dependent on the states of pin 6 (the threshold pin) and pin 2 (the trigger pin).

Here are the basic two rules to follow:

When the trigger pin is less than 1/3 of the supply voltage, the output pin will go HIGH.

When the threshold pin is greater than 2/3 of the supply voltage, the output pin will go LOW.

The other pin you may be interested in is pin 7 (the discharge pin), this pin is always the opposite state to the output pin.

In our circuit, knowing these rules, it becomes easier to understand what is going on!

When the circuit is first turned on, the trigger pin is lower than 1/3 of the supply voltage, so the output pin is tuned on. This begins to charge the capacitor connected between pin 6 and pin 2, and ground, through the variable resistor. When the capacitor charges to above 2/3 of the supply voltage, the threshold pin kicks in and pushes the output pin back off. This gives the capacitor time to discharge, lowering the voltage. Once the voltage is lowered below 1/3 of the supply voltage, the trigger pin pushes the output pin back on and the whole cycle starts again! We can change the time the capacitor takes to charge and discharge, by varying the 100K potentiometer, thus changing the duty cycle (amount of on time, compared to off time) of the signal. The diodes are there simply to make sure the capacitor charges through one side of the potentiometer, and discharges through the other.

While this is going on, pin 7 is doing the exact opposite to pin 3, giving us a useful oscillating signal!

We hooked an oscilloscope up to our circuit to show you more about what is going on:

variable voltage psu oscilloscope

Above you can see the output for pin 7 of the circuit, you can see it is turning on and off very quickly. According to the oscilloscope measurement, there is about 39uS (0.000039 seconds) between each complete cycle.


variable voltage circuit oscilloscope view

When we move the potentiometer, you can see the amount of time it is on decreases, compared to the amount of time it is off, this is called duty cycle! This effect is called PWM (pulse width modulation). The net effect of this is that to us the LED is ON for more of the time, and OFF for less; making it brighter. This may seem odd given pin 7 is off for more and on for less. But if you look at the circuit, the LED is going into pin 7, not out of, so when pin 7 is off, the LED is on, and vice versa!


power supply trace on an oscilloscope

Here we can see what the trace looks like when the LED is dimmer, pin seven is on for more of the time, and off for less, meaning the LED is off for more of the time, and on for less!


another oscilloscope view

Here you can see two waveforms on the screen, the top one is the feed from pin 3, the other is from the capacitor. You can see that when pin 3 is high, the capacitor is charging, until it gets to the trigger level, where pin 3 turns off, and the capacitor discharges. When the capacitor discharges to a low enough signal level, pin 3 turns on again, and the capacitor charges once more!


waveforms in sync

This final waveform shows the two overlaid, this is just to show that they are completely in sync!

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