So one day I was bored and had a few things lying around:
A pane of glass from a scanner
555 timer & passives
There was really only one logical thing to do to assist in my procrastination, put together an astable 555 circuit on the glass!
Through my time learning about electronics, I have come to realise that the 555 timer circuit, astable or monostable, is one of the first circuits anyone should make.
However for those who don’t know about it here is a short explanation of the astable circuit.
The 555 timer IC is a a circuit of over 40 components, including 25 transistors and 15 resistors, all printed on a silicone chip.
The circuit works by flipping the voltage states of different pins on the IC. Initially pin 7 is high and so the current flows though R1 & R2 to charge the capacitor. Pin 6 detects the high voltage build up on the capacitor and toggles pin 7 to be pulled low, this causes the capacitor to discharge through R2. While the capacitor is discharging, pin 3 is pulled low, turning off the output, however when pin 2 detects the low voltage on the capacitor, pin 7 is pulled high again, allowing the current to flow through R1 & R2 again.
And ofcourse there is some maths to work out the length of each high and low pulse for given component values, and thus the frequency as well.
f = 1 / ( ln(2) * C * ( R1 + 2R2 ) )
High = ln(2) * C * ( R1 + R2 )
Low = ln(2) * C * R2
And so with values of 1000Ω for R1 and 10KΩ for R2, and 100μF for C1, we get a high pulse of 0.76 seconds, and a low pulse of 0.69 seconds and a frequency of 0.69Hz (687 mHz).
Following the mod of the cheap lamp, I had a 12v supply lying around. I figured a good use of it would be to make a supply board for my Raspberry Pi and other devices I may want to attach to it.
The supply actually outputs around 13.4 v or so which can be attributed to the tolerances of components used. Regardless of output being greater than 12v, I can still use it with the two different regulators I ordered from Rapid, the L7805cv 5v 1A TO-220 package regulator, and the LM723 adjustable voltage regulator in a 14-DIP package.
Both regulators have a maximum input voltage of 40v, so the 12v supply will be just fine. However the supply outputs an Alternating Current (AC) signal, this can be converted to Direct Current (DC), which is needed for most general electronics, by passing the supply through a device known as a Bridge Rectifier.
The Bridge Rectifier
Moving On . . .
On my breadboard I first built the circuit to output 5v in order to power my Raspberry Pi.
This is a standard circuit found in the datasheet however C2 has a value of 100µF and C1 is equal to 10µF.
Oopps . . . Please do remember to put the capacitors the right way round, first time I’ve ever done it, but it turns out these capacitors don’t like 12v going in them the wrong way . . .
So after connecting the capacitors in the right polarity, and attaching a 7W 75Ω power resistor across the regulator’s output to load it, I attached the voltmeter to measure the output.
Using the 100µF and 10µF combination proved successful and outputted a solid 5.028v, however the datasheet recommends values of 0.33µF and 0.1µF. If anyone understands the reasons for the different values please do comment below because I am very curious as to why they both work.
Additionally I would be interested to know why the AC signal of the 12v supply distorts as seen below when the supply is under load.
So yesterday I had no lectures, so instead of spending my time working on assignments that I don’t have, I decided to go into the engineering lab and prototype the MIC2941 adjustable voltage regulator circuit.
The datasheet for the 2941 contains a example circuit with maths to go with it. This circuit should be capable of outputting between 1.2v and 26v, perfect for what I want to do.
So in the lab I found the components I needed, and set to work.
In order to calculate the values on R1 and R2 I used the equation provided, Vout=Vref(1+(R1/R2)) re-arranging so that, R1/R2=(Vout/Vref)-1. With this I then had a ratio that I could scale to any values I wanted.
Due to the output voltage being limited to between 1.2v and Vin – 1v I chose to aim for a output of 15v initially, because the power supply unit’s (PSU’s) my university have on workbenches only go up to 20v.
So with that value in mind I calculated R1/R2 = 11.195 to 3dp, therefore I selected a pre-set 2kΩ for R2 and a variable 22kΩ for R1. In theory I should then be getting approximate output voltages of:
14.76v at full rotation 7.995v at centre point 1.5375v at low rotation
As seen in the video below, with the reference voltage at 1.2 – 1.3v (poor resolution on the power supply’s behalf) and 19.9v on the input, the output voltage seems to fluctuate between 6.6v and then back up to 15.1v and back down.
My plan in the next couple of days is to hock it up to the oscilloscope and investigate the input current, which was limited to 0.5A by the PSU.
So the MIC2941AWT‘s have arrived! These are the adjustable 1.2A Low dropout voltage regulators which I will be using to provide a variable voltage supply. I will only be using 1 in my project which means I will have 2 spares.
To provide a readout of the supply voltage, current and power, I intend to buy a pre-packaged module off ebay. This module is sold in 3 different variations, the 5A module, 10A module, and the 20A module. The 10A module has the best current sensing resolution while still capable of sensing a large range and costs about £20.
Alternatively I can buy a Voltmeter and Ammeter module for £18 which also provides an adjustable output voltage and an aluminium heat sink.
We’ll wait and see which module I decide to go for.
After my mistake was pointed out that I needed fuses on my output terminals, here they are.