On this page I thought Iíd share some various nifty circuits. Some of them are pretty common and standardised and Iím not aiming to put any specific restrictions on their use. If you do use them, itíd be good if you mentioned my site or whatever, but you donít have to.
Firstly, Iím not an expert in circuit design by any means. Far from it, Iíd say. It might be something I started with probably in the late 1980s, but really I just have a dabble now and then with stuff that might interest me to make, and usually the final result is what Iím after. It hasnít been a primary interest in my life.
If youíve never done anything with electronic components before, then this page isnít the best place to start. You should know about soldering, perhaps using bread boards, and PCBs. (Printed circuit boards). You donít HAVE to get a PCB designed to suit. You could just use a prototype board in most cases. Also, reading a circuit diagram is also something you should be able to understand. Iíve tried to use standardised symbols as much as possible to avoid confusion.

Audio Level Indicator

This is only illustrated for one audio channel. If you want it for stereo, you need to make a duplicate of all the parts, although you could probably share the large filter capacitor on the power supply.

Years ago I got Dick Smithís Fun Way 2 audio level kit. It was based on discrete components, and didnít use any integrated circuits. It was also made to be connected in parallel with the speakers on your amplifier. Well, after reading about the LM3914 I.C. back in 2020 or so, I thought I would try making my own level display on that, since it seemed to cut out a lot of excess components. That kind of got delayed until this year in 2022, since I partially lost interest, and had more important things to spend my money on. I also wanted something much more sensitive, which would respond to the lower volume levels of my PCís sound card, especially with my noise cancelling headphones, which require very little amplification, since they do their own amplifying anyway. So this circuit above is designed for such use.
The LM3915 is logarithmic, instead of linear, like the LM3914, (which is more for showing voltage levels of batteries,) and aimed for audio related readings anyhow. You can use the LM3914 or LM3916 in the same circuit, but they behave slightly differently. These I.C.s are actually classed as obsolete, as the original manufacturer pulled the plug on them, as it were. But you can still easily get them, and other companies make compatible versions too. The LM386 shown in the circuit, is a low power, 1 watt amplifier. Iím using it to boost the input volume so that the LM3915 responds at lower levels.
I like to know the reason for components in circuits, so Iíll try to explain what I can. The 1 ĶF capacitor whose positive side connects to pin 5 of the LM3915 I.C. is present to hold a charge to keep the L.E.D.s lit long enough for you to see them. This is important for the dot mode especially. The 100 kΩ resistor in parallel with it, I think is to bring down its charge a bit quicker. That I wasnít too sure about, but it was shown in several other circuit designs such as this. The input socket at the bottom left can be connected in parallel with another to provide you with a way to connect your headphones. But remember, this circuit is only for mono use. Youíll need to build another copy of it for stereo. But they can share the same power source and 220 ĶF filter capacitor. If you run it from a battery, this capacitor may not be quite so necessary.
L.E.D. 1, which represents the lowest sound level, connects to pin 1 of the LM3915, and then L.E.D. 2 connects to pin 18, and they work backwards, down to pin 10, which is the loudest level.
The resistor which connects from negative to pins 6 & 7 is usually shown in other peopleís circuits as around 1.2 kΩ, but as I wanted my super bright L.E.D.s to be dimmer, I cranked it up to 4.7 kΩ. This is the resistor which sets the LM3915ís current limitation to the L.E.D.s. You can reduce its heat level further, by connecting a low value resistor in series with the common side of the L.E.D.s and the positive connection of the power source. I didnít show them in this circuit, but Iím going to add extra resistors per L.E.D., since Iím using super bright ones in varying colours. Some need more resistance than others to equalise the brightness. As an added bonus, this will also allow the components to last longer. The LM3914, LM3915, & LM3916 I.C.s can only dissipate a certain amount of power before they start running too hot. I think itís something like 660 mW.
The 10 ĶF capacitor which connects from pin 1 to pin 8 of the LM386, is to set its gain to 200. Without it, the gain is 20. The output from pin 5 of this I.C. runs through to a 10 ĶF capacitor, which I think works kind of like a separator, the same as the one at the input socket. Itís recommended that the diode in the centre, is a signal type, like the 1N4148. I didnít have any of these, but I found that the base to emitter of a NPN transistor worked just as well.
Pin 9 of the LM3915 is connected to the positive side of the power supply to make the I.C. run in ďbarĒ mode, which means all the L.E.D.s from 1 up to the highest current sound level will be lit. If you disconnect it, in my case with a switch, it will run in ďdotĒ mode, which means only 1 L.E.D. will be on at the top end of the sound level.
You can run the LM3915 with an audio source directly into pin 5, but the result from the L.E.D.s is nowhere near as good. You really need the diode and capacitor on pin 5. The diodeís purpose is pretty much to allow the capacitor to charge, because without it, it wonít work. I discovered this myself, when trying different component configurations.
ē Iíve made some updates to this, since it didnít work properly. Shame on me! Anyway, the changes are in deep green. The extra resistor prevents the level of L.E.D.s from ďfadingĒ away to nothing. I had issues with my old capacitors which didnít show this as much as when I replaced them with new ones, making me wrongly assume the circuit was correct. I also reduced the 10 ĶF capacitor between the amplifier I.C. & diode with a 1 ĶF one, which is more responsive to the treble parts of your music.
ē Iíve finally got this completely assembled, and I should probably mention that a 500 kΩ variable resistor might be more appropriate for louder volume inputs.

L.E.D. Flasher

Next up, Iím going to look at a way to flash some L.E.D.s. This is known as a flip-flop / multivibrator circuit, and it goes back to around 1919. (Back then they used valves instead of transistors of course.) Itís been done in many electronics books, and itís rather easy to find examples of on the Internet too. You could use it for crossing lights on a model railway, for a warning that a door may be opening, model police cars or many other fun purposes.

Just 2 transistors, 2 L.E.D.s, 4 resistors & 2 capacitors.

The circuit basically ďunbalancesĒ itself due to component tolerances. When itís turned on, the capacitors will charge up, but even though theyíre the same value, one will charge up first, and cause the transistor opposite it in the circuit to turn on, which cuts off the other capacitor from charging. Thereís a bit more to it than that, but basically it starts a cycle, and the L.E.D.s will flash from one to the other. Itís important that the components are closely matched, or the flashing will be uneven. Using different L.E.D.s may even make it seem like one doesnít fully turn off at all. By decreasing the voltage of the power supply, the flash rate will increase. This will also happen if you decrease the value of the capacitors. In fact, if their value is small enough, the circuit could be modified to use a low power speaker instead of the L.E.D.s, which would then make a tone. Something like 0.05 or 0.1 ĶF in value. In this situation they donít need to be polarised capacitors either. Changing the value of the 10 kΩ resistors, or just one of them, will also alter the speed. Some circuits show this with a variable resistor too.
The circuit shown is for PNP transistors, but if you want to use NPN, simply flip the polarity of the power supply, L.E.D.s and capacitors.
If you want to add more L.E.D.s in parallel, keep in mind how much current your choice of transistors can handle. (A BC558 type can handle 100 mA & dissipate 300 mW.) If you increase the voltage, you will need to change the 390 Ω resistors too. (On 12 V, with L.E.D.s rated for 2 V at 20 mA, you could use a 510 Ω resistor, since thatís the closest value.)

Choosing Resistors For L.E.D.s

Light emitting diodes only require a very small amount of power, and using too much will cause them to overheat, melt and generally stink with a nasty pong. The basical mathematical formula to choosing a resistor which will connect in series with a L.E.D. is: (Power Source Voltage - L.E.D. Voltage Rating) ų L.E.D. Current Rating ◊ 1000. So using that for a 5 V power supply and a 2 V L.E.D. rated at 20 mA, you would work it out like so: (5 - 2) ų 20 ◊ 1000, and the answer would be 150 Ω. Use a higher value resistor if the value you want doesnít exist. Remember that this is for the maximum specifications of a L.E.D., and if you run them underpowered, theyíll last longer.

A Regulated DC Power Supply

This circuit is designed to connect to your buildingís high voltage power supply, so if youíre not confident in making something for that, donít do it. Remember that coming in contact with your houseís 240 V power supply is most often lethal, so everything on that side of the circuit needs to be properly insulated. If you kill yourself, or somebody else, then youíre responsible! Donít take shortcuts, and treat this seriously.
If you have some old transformers lying around, you can easily turn them into a DC power source for various electronic projects. Otherwise, if you donít want to build something yourself, youíd be better off buying a little regulated switch mode power supply in a ďplug pack.Ē

This is a basic setup using a 12 V transformer and producing a fixed, 9 V output.

In the example, is a 12 V transformer. To the right of it, the 4 diodes make up a bridge rectifier. Diodes have a voltage drop of 0.6 V, and taking into account power fluctuations, a little more oomph is recommended for a 9 V output. You can make your own bridge rectifier using 4 power diodes, or you can buy a single rectifier, which basically has 4 diodes inside it. Some of them are cylindrical in shape, square or even rectangular. They will have a wavy symbol on the pins which connect to the AC input, and a + & - for the DC output. Be sure to check what level of current the rectifier can provide. Some may be 1 A or even 3 A, or a lot more.
The capacitor to the right is for filtering. (Itís an electrolytic type.) Even though weíre directing the positive and negative parts of the AC input with the rectifier, you still get a ripple. On the right of that is a 7809 voltage regulator, which is designed to go on the positive side. These should have a heat-sink, and theyíre usually only rated for 1 A. Apparently you should not bolt these to an earthed case, nor should the negative part out from the rectifier be earthed. You should also keep the filter capacitor as close as possible, or use a second low value ceramic capacitor right next to the voltage regulator, such as around 0.1 to 0.2 ĶF or so. The 2nd capacitor is recommended to be a ceramic type, and itís not polarised. This just does any final filtering of disturbances in the power.
There are other voltage regulators too, such as the 7805, which does 5 V, and 7812, which does 12 V. Your input voltage should always be slightly higher than the intended output voltage, but each regulator has a maximum rating too, which you should look up.
Be sure to check your transformerís power rating and output voltage as well, if you donít know it. You can check the voltage with a multi-meter. (Start from the highest setting and work down.) If youíve been salvaging transformers from old appliances, and think one may produce an even higher voltage on the secondary side, it wonít be suitable for low voltage circuits. Be very careful if youíre looking into microwave oven transformers. This is an area I donít know a lot about, and I think some of them generate very high voltages.

L.E.D. Chaser

Who doesnít love a good chaser circuit?! Iím using the traditional 555 timer & 4017 decade counter, but with a Toshiba ULN2803APG darlington transistor array, so you can run up to 500 mA per channel, and not cook the fig out of that poor counter! (Assuming your power supply can handle such a load.) The 4017 I.C.s really arenít rated for running high loads of multiple L.E.D.s. You can get away with 2 mA ones perhaps, but not 4 L.E.D.s running in parallel, with each rated at 20 mA. The ULN2803APG, & similar types, also have diode protection built in, so you can also have them run motors, relays and solenoids. You can slap another one in if you want all 10 outputs from the 4017 I.C. Or just use 2 transistors. The output pins, in order, from the 4017 I.C. are: 3, 2, 4, 7, 10, 1, 5, 6, 9 & 11.
There are many circuits which do various effects with multiple L.E.D.s, but this type of circuit will only show 1 L.E.D. lit at any one time. Unless you connect 2 L.E.D.s together, but there is only 1 active output at once. My circuit limits the L.E.D.s to 8, because thatís all the ULN2803APG I.C. can do. Other similar darlington transistor arrays, such as the ULN2003N, only have 7 outputs. Because Iím not using the whole count of 10, pin 9 of the 4017 I.C., (which is also the 9th output,) is connected to pin 15, which resets the counter. If you donít do this, thereíd be a gap of 2 counts between the last L.E.D. coming on, and the 1st one.

Itís a bit messy, but if you want to see how the connections appear physically, then this is how they go. I coloured certain wires so theyíre easier to follow.

The speed of the chasing is controlled by the variable resistor on the left. Itís good to use 2 L.E.D.s in parallel so they look like theyíre following each other. (That would be 16 in total for this particular circuit.) The 3.3 ĶF capacitor also has an effect on the speed. The capacitor on the right side is just to act as a filter. If youíre running it from a 9 V battery, you could probably do without it. If you want to up the voltage, youíll need to change the 390 Ω resistor on the right to a higher value. The I.C.s should be happy on 12 V, but I wouldnít crank things too high.

Dual Switch Light Circuit

This was something I saw in hotels in Britain when I was there in 2002. Theyíd have a switch by the door, and one by your bed. Either switch could reverse the effect of the other one.

This is the basic principle, and it could be used for high voltage, A.C. or D.C. projects.

As you can see, it just uses 2 SPDT type switches in series with the light. But it could be adapted for other purposes too. It is possible to add a 3rd DPDT switch, which swaps the connections of the right hand side parts of the other switch connections. Maybe you could also add a 3rd switch in some other fashion too, but that was all I could work out.

Relay Hold Circuit

This simple circuit demonstrates how you can use 2 push buttons to turn a relay on and off.

Obviously push button 1 is normally open, and push button 2 is normally closed.

By pressing push button 1, the relay will then turn on, and its own switch will keep it turned on. Pressing push button 2 will turn it off again. This is only to show an example ó youíd probably want a relay with 2 sets of switch contacts to make it practical for turning other stuff off / on. If youíre using the relay on a D.C. power supply with semiconductors, like integrated circuits and transistors, itís wise to connect a power diode across the relay coil with its anode going to negative and its cathode going to the positive side. (Put it back to front and itíll effectively make a short circuit.) This directs any residual current in the coil back into itself.

Polarity Changer Circuit

Using a DPDT switch, you can flip the polarity of a DC power supply.

This is shown using a slide switch, but it doesnít matter what kind of method it uses.

With the switch set to the left, only LED 1 will be on. When you move it to the right, only LED 2 will light up. These are only here as an example. You could use such a circuit to test 2 pin bi-colour L.E.D.s, but you may have other reasons for such a circuit too. One other practical purpose would be for DC motors.