One Transistor Automatic Battery Charger Circuit

A very simple single transistor automatic battery charger circuit is described in this article, which uses just a single transistor for the voltage detection as well as for automatically disconnecting the battery from the supply when it gets fully charged.

Circuit Description

As shown in the diagram we can see a straightforward configuration where a single transistor is connected in it’s standard operating mode. 

The circuit functioning may be understood with the help of the following points:

Considering the battery to be charged is a 12 volt battery, we know that it is advised to charge the battery until it reaches between 13.5 and 14 volts.

The transistor base voltage is adjusted using the preset P1, such that the transistor just conducts and operates the relay at around 14 volts.

This adjustment becomes the high voltage trip point of the circuit and is used to switch OFF the charging voltage to the battery when it gets fully charged or its voltage reaches around 14 volts.

The lower trip point of the circuit cannot be adjusted as this circuit is too simple and does not incorporate the low voltage detection feature.

However the transistor is itself equipped with a switch OFF feature in case its base voltage becomes too low.

Typically a general purpose transistor like the one which is shown (BC547) when adjusted to switch ON at 14 volts may have the lower threshold of around 10 volts, when it might get just switched OFF.

This wide voltage difference between the high set threshold and the lower natural threshold is because of the involved big hysteresis with the design.

The lower threshold of 10 volts is dangerously low and we cannot wait for the circuit to restart the charging process until the battery voltage falls to this dangerous 10 volts level.

 Allowing the battery to discharge down to 10 volts can make the battery flat permanently and reduce its life.

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Therefore to eliminate this issue the circuit needed to somehow reduce the hysteresis level. This is done by introducing a couple of diodes at the emitter of the transistor.

We know that normally a 1N4007 diodes would drop around 0.7 volts across it and two if them would make a total of 1.4 volts.

By inserting the two diodes in series with the emitter of the transistor, we force the transistor to switch off 1.4 V earlier than its normal specified limit of 10 volts.

Therefore now the lower operating threshold of the circuit becomes 10 + 1.4 = 11.4 volts, which may be considered just OK for the battery and for the automatic restart of the charging process.

Having both the thresholds updated as per the standard charging requirements, we now have an automatic automotive battery charger that’s not only cheap to build but also smart enough to take care of the battery charge conditions very efficiently.

Parts List for the proposed one transistor automatic battery charger circuit

R1 = 4K7

P1 = 10K preset,

T1 = BC547B,

Relay = 12V, 400 Ohms, SPDT,

TR1 = 0 - 14V, current 1/10^th of the battery AH

Bridge diodes = Equal to the current rating of the transformer,

Emitter diodes = 1N4007,

C1 = 100uF/25V

Simple, Reliable Infrared (IR) Remote Transmitter/Receiver Circuit

Ordinary IR remote control circuits have one big drawback, they easily get disturbed by stray external frequencies, and thus produce spurious toggling of the load. The circuit discussed here eliminates this issue, and detects only the specified IR frequency from the given remote transmitter unit.


In one of previous posts I have discussed a simple IR remote control circuit which operates quite well, however the circuit is not completely immune to external electrical disturbance generations such as from appliance switching etc. which results in false operations of the circuit causing lot of annoyance to the user.

The circuit design included here efficiently overcomes this problem without incorporating complex circuit stages or microcontrollers.

The solution comes easily due to the inclusion of the versatile IC LM567. The IC is a precise tone decoder device which can be configured to detect only a specified band of frequency, known as passband frequency. Frequencies not falling within this range will have no effect on the detection procedures.

Thus the passband frequency of the IC may be set precisely at the frequency generated by the transmitter IR circuit.

Shown below are the Tx (transmitter) and the Rx (receiver) circuits which are set precisely to complement one another.

T1 ad T2 along with R1, R2 and C1 in the first Tx circuit forms a simple oscillator stage which oscillates with a frequency determined by the values of R1 and C1.

The IR LED1 is forced to oscillate at this frequency by T1 which results in the transmission of the required IR waves from LED1

As discussed above, R5 of IC2 in the Rx circuit is adjusted such that its passband frequency precisely matches with that of LED1 transmission output.

When the Tx IR waves are allowed to fall over Q3 which is an IR photo transistor, a subsequent order of varying positive pulses is applied to pin#3 of  IC, which is basically configured as a comparator.

The above function generates an amplified output at pin#6 of IC1 which in turn gets induced across the input or the sensing pin out of IC2.

IC2 instantly latches on to the accepted passband frequency, and toggles its output at pin#8 to a low logic level, triggering the connected relay, and the preceding load across the relay contacts.

However the load would stay energized only as long as  Tx stays switched ON, and would switch OFF the the moment S1 released.

In order to make the output load latch and toggle alternately, a flip flop circuit will need to be employed at pin#8 of IC2.

Parts List

R1 22K 1/4W Resistor

R2 1 Meg 1/4W Resistor

R3 1K 1/4W Resistor

R4, R5 100K 1/4W Resistor

R6 50K Pot

C1, C2 0.01uF 16V Ceramic Disk Capacitor

C3 100pF 16V Ceramic Disk Capacitor

C4 0.047uF 16V Ceramic Disk Capacitor

C5 0.1uF 16V Ceramic Disk Capacitor

C6 3.3uF 16V Electrolytic Capacitor

C7 1.5uF 16V Electrolytic Capacitor

Q1 2N2222 NPN Silicon Transistor 2N3904

Q2 2N2907 PNP Silicon Transistor

Q3 NPN Phototransistor

D1 1N914 Silicon Diode

IC1 LM308 Op Amp IC

IC2 LM567 Tone Decoder

LED1 Infa-Red LED

RELAY 6 Volt Relay

S1 SPST Push Button Switch

B1 3 Volt Battery Two 1.5V batteries in series

MISC Board, Sockets For ICs, Knob For R6, Battery Holder

RELAY 6 Volt Relay

Cellphone Controlled Car Starter Circuit

The post presents a simple cellphone triggered remote control circuit that can be applied as a cellphone operated remote car starter. The unit would cost less than $20 to build.


I have already covered quite a few interesting cellphone remote control circuits in this blog, all of which can be implemented to control or toggle some electrical equipment remotely using ones own cell phone, exclusively.

The basic cellphone controlled relay circuit stage involved in all the previous circuits can be also effectively utilized for starting the ignition system of a vehicle, through the owners cell phone. The schematic for the same may be witnessed below and may understood with the following explanation:

The design is basically a transistorized audio amplifier circuit, which is positioned for amplifying the assigned ringtone from the adjoining cell phone modem. The cellphone shown with the circuit stays permanently attached with the circuit and forms the integral part of the whole system.

The diagram shows a NOKIA 1280 cellphone as the modem, however any cheap cellphone may be employed for the purpose provided the cellphone includes the "assign tone" feature discretely for the particular selected numbers.

The number of the owner or the user is first stored and assigned with a suitable ringtone available within the cellphone modem so that the modem responds only to the owners phone and not to any other irrelevant numbers. The default ringtone of the modem is set to "empty" in order to mute all other unwanted calls.

When the owner calls the moden cellphone, the ringtone is detected by the circuit and amplified to a level sufficient for the relay to get energized. The relay stays energized for so long as the call remains connected.

Since the relay contacts are configured or integrated with the ignition switch of the car, immediately triggers the ignition system of the vehicle starting the engine and the whole system.

A feed back from the activated alternator makes sure that the relay is instantly shut off irrespective of the call duration from the owner's cellphone.

The car ignition thus is able to start without the owner or the driver having to get inside the car and go through the manual operations. The car starts remotely through the owner's cell phone, a fail proof and a foolproof procedure yet as cheap as anybody can think of.

Parts List for the cell phone controlled car ignition starter circuit

R1 = 22k
R2 = 220 Ohms,
R3 = 100K,
R4,R6,R7 = 4K7
R5 = 1K
R8 = 33K
R13 = 100 ohms,
T1, T2, T4, T5 = BC547
T3 = BC557,
C1 = 0.22uF
C2,C3, C4 = 100uF/25v
D1, D2 = 1N4007
L1 = 40 mH coil, example: a piezo buzzer coil will do.
diode = 1N4007
Relay = 12V/SPDT
Modem = NOKIA 1280

The charger section is shown in the diagram, and needs to remain connected permanently with the attached cellphone modem.

Three Phase Moter Star Delta Connection