Wednesday, January 31, 2018

Free Ebooks

Huge amount of free books are available in the internet and in many cases sites of those free books are not that great. Often times there are limits on how many you can download in a certain period of time or maybe they are not that well designed and many other issues. I have collected a good number of free books from many different sources which I think will be a good idea to share.

Note: Only old and free books are available here, no piracy is involved.

Link To Free Books.

Writer and Books-

1. Agatha Christie
2. Enid Blyton
3. Franz Kafka.

Thursday, January 25, 2018

Single Cell NiMH Battery Charger

With the advent of lithium ion batteries Nickel Metal Hydride batteries are uses less and less yet they can be used in many scenarios and to use them you have to charge them. A smart charger would be perfect fit for that purpose but those chargers are often very pricey so if you don't have that many cells to charge you can make a simple charger and just it a day. Here is how you can make such a device.

Parts List:


1 10Ω 1W
1 1KΩ


2 470μF 25V


1 TIP 127 PNP
Heat sink


4 1N4007
1 Green LED


220V - 9V 1A

Battery Holder:

1 For Size AA.


How does it work?

From the left the transformer steps down the AC line voltage to 9V which is then rectified by the four diodes and smoothed by the capacitor next to them.

The Green LED plays a very crucial role and keeps the base of the Transistor roughly at 2.6V. After the 1.2V drop we get around 1.4V to charge the NiMH cell.

The charging current is around 140mA.

Tuesday, January 23, 2018

Automatic Solar Garden Light

This will be a very simple but practical circuit for lighting up your garden. You can make this circuit in weather proof packaging and forget about it. Occasionally you might have to change the NiMH battery used in the circuit.

The automatic part will depend on the amount of light the solar panel is getting. If it does not get enough light it will turn on its own light and as soon as it gets light again it will automatically turn off and start charging the battery inside it.

Parts Required:


1 4.7Ω
1 47Ω
1 1KΩ
1 2.2KΩ


2 2N3904 NPN


1 47μF 16V


1 1N4001
1 White LED




3 AA NiMH 1000mAh
Battery Holder

Solar Panel:

5V 250mA Solar Panel


 How Does it work? 

To power the circuit during night there is a NiMH battery pack which consists of 3 cells, giving us about 3.6V. To charge this battery a 5V solar panel is used. The 4.7Ω resistor will drop some voltage across it to make sure not to push the battery too hard while charging. The 4001 diode will stop any back flow from the battery so that the automatic light up part can work.

The automatic part is pretty simple here. When there is light solar panel will provide enough current to charge the battery and keep the first transistor(from the left) On as it is getting some current from the panel. However as soon as the sun goes down the solar panel will no longer provide current so the aforementioned transistor will turn off causing the second transistor(again from the left) to turn on and light up the LED from the battery pack. When the first transistor was On, it kept the base of the second transistor virtually grounded so the second transistor was Off.

 Visit My Full Blog.

Saturday, January 20, 2018

Simple USB AA/AAA NiMH Charger

This post is about a simple USB powered Nickel Metal Hydride charger with an in depth analysis of how it works.

Parts Needed:


1 18Ω 1W
1 22Ω 1/2W
1 180Ω 1/4W


1 100μF 16V


1 1N4007

Battery Case:

For AA and AAA.


1 Micro USB 




How it works?

As the name suggests it is a very simple design so there will not be a lot of parts used. Here is the run down on what each of the parts are doing.

The circuit is powered by micro USB 5V. The first 100μF capacitor is there to just smooth things out. This capacitor might not be necessary. 

Next comes the Diode, which blocks any back flow from the cells and kind of isolate each cells.

The SPST switch is for selecting the Charging Current. Open for AAA about 80mA, Closed for AA about 175mA. If it is Open both the resistors will be used in series and total charging current will be around 80mA which is okay for AAA cell. When the switch is Closed it shorts the 22Ω resistor so only the 18Ω resistor is in the circuit now which causes the total current to rise to around 175mA which is okay for AA.

If you have a high capacity AAA however that can accept high amount of current you can charge AAA while the switch is in AA position. However if you have a low capacity AA and want to charge it slowly you can use the AAA position for that as well. (I'm talking about switch position). 

The LED is there to indicate the cell is connected and charging. If you disconnect the cell it will stop glowing. 

If you want to charge multiple cells just make the same circuit twice, you might use a common 220μF capacitor in that case.


This one is a dumb charger so won't do any fancy thing. It will keep on charging as long as the cell is in there. So you kind of have to measure the time yourself. For 700mAh cell, in AAA settings you will need around 12-14 hours to charge. For 1300mA AA in AA settings you will need around 8-10 hours. These time measurements are for fully drained cells.


Visit my full blog.

Wednesday, January 10, 2018

Mains Powered Simple Blinking LED

This diagram might not be that useful for what it is but it teaches the use of an interesting component called DIAC.

So what this circuit does is blink an LED and can be hooked up directly to AC main line and it needs only a few components.


1 47kΩ
1 220Ω

1 47µF 63V

1 1N4007

1 DB3


How Does it work?

When power is turned on the 47µF capacitor starts to charge via the 47kΩ resistor and 1N4007  diode. As soon as the voltage hits the break over voltage of the DIAC DB3(Which is 32V, more information in the datasheet linked below) it starts to conduct and the LED lights up.

As the current is passing through the LED now it empties the charge stored in the capacitor faster than what the 47kΩ resistor can provide so soon enough current through the DIAC falls to a point when it will just stop conducting. Thus the LED turns off.

Then the whole process repeats again giving us a blinking LED.


1. Remember that even if the DIAC was triggered with a certain voltage it will keep conducting unless the current through it falls to a certain value. Once triggered, even if the voltage falls below break over voltage it will still conduct.

2. 47kΩ resistor and 47µF capacitor can be altered to control the blinking frequency.

3. This circuit is not mains isolated so to avoid electrical shock do not touch any part of the running circuit.


1. Datasheet for DIAC DB3.
2. More about DIAC.
3. Visit my full blog.