Zero drop dusk to dawn solar charge controller by Sandeep

When you work with solar panels, you don’t just need a panel to supply power, battery to store power and lights to glow, you will always need a good charge controller. The charge controller is a device which keeps status of battery voltage, solar voltage, etc and charges the battery in a manner that the maximum efficiency is achieved.

There are many kind of charge and supply controller which work as their design. Today I’m going to introduce you a ‘Zero drop dusk to dawn solar charge controller”.

From the name, we can say that the charge controller features a Zero drop of voltage between the battery and solar when charging. This feature eliminates unnecessary heat and gives more efficiency compared to other models. The charge controller is made by my friend Sandeep Kumar who is an electronics engineer by profession and he works on the solar technology. He has designed the charge controller in such way that it detects the solar voltage and determines the dusk and dawn and takes appropriate strategies to charge the battery or changeover mains supply. the device features LED status indicators to indicate battery charging, full charge and mains cut-off.  With minor adjustments the kit can be used for 6,12,24 or 48V solar systems.


You can email Sandeep at sandysplash230 @ for details on design or for purchasing the kit along with diagram, hex code, tech transfer, etc options.

Inside a $10 phone

Last week I bought a cheap china phone as my primary phone went to service center for 3 days.
It’s this:
I got the phone for Rs.500 (~US$8.4) including battery,charger,earpiece. And after my primary phone was back from service, I had the good time to open the phone and see what’s inside.

It was a lightweight phone which looked similar to Samsung E2232 and had many useless things in it.
1. Camera: Worst camera ever seen, 96×68 res. What do you expect in such phone ? 😛
2. Media playback: It managed to play MP3/MP4 over TF card but sound or video quality will destroy your ears.
The two good things to mention:
1. The battery held the initial charge for the 3 days.
2. It has provision of using two GSM SIM at the same time.

So, it has a generic PCB. You can see two LCD connectors, extra switches (probably same board with different ROM for different model) and there’s even a 4pin touchscreen connector at the top.


On the back of the PCB there is almost no parts as compared to ordinary Nokia/Samsung phone’s motherboard. There is one small IC below battery compartment which looks like power management IC, one RDA8851A CPU, and one IC section shielded which is obviously the GSM unit. There’s connction pads provided for vibrator, additional LED for torch, and a 6pin programming interface connector.
Oh yes, there’s a hanging wire for Bluetooth antenna also. 😛



Happy New year & Intro:BuyCircuitsOnline


We wish all our readers a happy and prosperous new year. We wish to do more experiments/engineering this year.

In this new year occasion, a new website is launched from our side. BuyCircuitsOnline. This will be our sales channel where you can buy kits and products developed or distributed by the team at Circuits DIY. Our payment system supports all major Indian cards/netbankings and International cards. We don’t expect any huge sales through the channel, but we needed to separate sales from the main website so the daughter website is opened.

Image credit: christmasstockimages under CC-BY license

USB li-ion charger built by Jamie


Jamie Navarro built a USB li-ion charger on breadboard. Now he is working to get it fine tuned to accurate voltage levels.
He scored a free PCB for this. Wish him success this time.

Failed Prototype: VAmeter v3

Just a few days ago the PCBs for VAmeter v3 came in.

VAmeter is a simple board that connects to your existing power supply and shows the volatge, current, etc.

Upon building, the major fault was discovered. There was no connection to the input voltage so the unit is unable to measure the input voltage. It is only able to measure the current.

However, This can be fixed by adding two more flying resistors to the board, but that makes the PCB design a failure.

Faulty schematics

Usb li-ion charger build

This is the completed PCB of the USB li-ion charger circuit. Read the rest of this entry »

Low voltage cutoff circuit for Inverters

Running inverters on battery drains the battery. This decreases the battery voltage. To regulate output voltage and to make it independent(upto a certain level) we use PWM based drivers like the SG3524 based one. Lets consider the following example:

  • Battery capacity: 100Ah
  • Battery voltage: 12V
  • Inverter load: 500Watts
  • Inverter efficiency (avg): 80%.

Now when the battery is full, it will give some 13.5V. When the above inverter is in action at the load, it will draw near 46.3A current. Drawing this much current will make the battery drain in less than an hour. Assume that after 50mins the battery voltage is 10.5V(almost discharged), then the PWM based one will draw more current like 59.52A. At this time the battery has low charge but the inverter is eating max current out of it. This can lead to extremely discharged battery if there is no low voltage cutoff protection. It should be mentioned that this types of discharging decreases battery life.

Firstly, low voltage cutoff circuit should have two threshold volts, one for stop and one for start. If it has one volt threshold say,10.5V, then when battery will reach 10.5V it will cut the load, but cutting load will make the battery voltage higher and the load will be on and the load will make voltage lower than 10.5V again and the cycle will go on. For this reason we need two threshold  voltages. For example consider the cutoff voltage is10.5V then when the battery goes below 10.5V the load is cutoff, after load if disconnected the battery voltage will increase by 0.1~0.3V, so we need to make the power on threshold voltage to 11V.

So, the first challenge is to make a device with two threshold voltages one for cutoff another for turnon.

Secondly, an average inverter of 500watts(example above) eats 50-60Amps of current. If we want to cutoff the supply by really, we’ll need atleast 60A rated relay which is too expensive. Also, if we use MOSFET with typical RDS(on)=0.015~0.020 Ohm , then at the full load we’ll loss near 1V which is near 8% of energy also that will make 50Watt of heat in the MOSFET. So, we need not to dut the supply line for the power stage, bu we need to cut the supply for the driver stage. The driver circuit normally sends positive pulses to activate MOSFET or BJT in the driver section, so we need to cut the supply line (positive) of the driver circuit. Now, a typical driver circuit eats 30mA current, so we can make the switching through a relay as well as through a regular transistor.

The sketch should be as follows.

Work on circuit in progress and will be updated soon.


USB li-ion charger revision

An year ago, I’ve shared a circuit of USB li-ion charger based on LM358opamp. The circuit had some problems as reported by commentators that inclide wrong direction of LEDs.


  • Charging via mini-USB connector which is very common.
  • Charging status display by LED
  • Simple circuit by using opamp, resistor, and not by any complex dedicated IC or micro-controller.
  • Charges completely drained (0V) battery packs.
  • Max charging current 500mA(limited by USB supply), depending on battery.

Here’s the corrected schematics and the board design of the circuit.



Boards are available for buying.



Bench PSU made by ARTESYN NLP40-7629J SMPS

Few months ago I ordered ARTESYN – NLP40-7629J – SWITCH MODE POWER SUPPLY from element14. Including shipping and taxes, it took nearly Rs. 3500 (~USD $70) to reach me. Have a look at the SMPS in the pictures below.

Read the rest of this entry »

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H Bridge inverter power stage with passives


The above circuit I got from replies from edaboard. I had created a thread there on how can I drive a 2 pin primary transformer with H bridge configuration with my SG3524 PWM circuit. I got many replies to use dedicated driver ICs, suggestions to use floating high side gate driver, etc. But I simply needed a solution to run it with circuit made of passive and discrete components and alertlinks gave me this circuit.

This circuit generates a 24V supply from the 12V battery voltage and makes the required high voltage to drive the upper FETs.

The 24V supply is made by a charge pump circuit. In the first two signals from teh SG3524 IC,the alternate 100uF/25V capacitors get charged and it works.