Wednesday, November 26, 2014

Simple Automatic Car Battery Charger

This is modified version of commonly available automatic car battery charger system. I obtain original layout of this charger from one of the commercially available Chinese car battery charger and modified some of the sections of original schematic to improve the performance and stability of the system. This charger is mainly based on LM311 comparator and 2 ×12V 4A transformer.

Final view of automatic car battery charger

All the components of this car battery charger are commonly available in the market. Ammeter in this system must be withstand for minimum of 8A current. Low cost ammeter can also be used with suitable shunt. For the relay it is highly advisable to use some high quality unit. During our prototyping we mainly use 12V Omron and Panasonic realys with this system. At the test stages we observe that some of the cheap relays may start to oscillate heavily with this system.

Automatic car battery charger finished prototype

Under normal conditions TIP142 and 7805 regulator in this system may not need any heatsinks.

This unit is specifically design to charge 12V lead acid car batteries and it is not suitable to use with other battery types. 

Modified schematic of this car battery charger is available to download at

Saturday, November 15, 2014

Homebrew 5 element Yagi antenna for 144MHz

This is 5 element Yagi-Uda antenna for 144MHz amateur radio transmissions and receiving. This antenna is design using commonly available TV antenna parts. The CAD drawing of this antenna is available to download in my pages and all the measurements in that diagram are in mm (millimeters). The total boom length of this antenna is around 1500mm and maximum span is 1016.10mm.

3D visualization of 5 element Yagi antenna

After the construction I test this antenna on 145.625MHz and got SWR of 1:1.3 with quiet lengthy, standard 50Ω RG-58 cable. With high quality shorter cable I archive SWR of 1:1.1 - 1:1.2.

Thursday, November 6, 2014

Simple CMOS FSK modem

This project is about 4046 PLL base simple FSK modem. Under given configuration this modem send digital signals over two frequencies and which is switching around 378kHz and 226kHz. In this system, modulator uses VCO of 4046, and demodulator is based on close loop mode of 4046 phase comparator and external LPF assembly.

This FSK modem is specifically design to transfer digital signals over mixed signal lines and it can directly use with any slow digital interfaces like UARTs. This system operates reliably under 10kHz range and we test it with standard TTL inputs.

1kHz Input signal and demodulated signal

1kHz Input signal and modulated signal

The given PCB of this system is based on SMD components but it can also construct using standard through-hole components.

During our prototyping we test this system with ATmega series microcontrollers and TTL signals sources. Because of 4046 CMOS implementation this unit can operate in between 3V to 12V by without using any external regulator(s) or level shifters.

Schematic diagram and PCB design patterns of this project are available to download at