This Project is for the Circuit designers. Lot of projects are experimented in Bread board and Perf boards before finalising the project. So these projects need a power supply to test. Usually a 9V PP3 battery is used to power the bread board. So here is a compact Portable Power supply unit for the Bread board experiments. The Power supply includes
1. A charger indicator for the 9V rechargeable battery
2. A battery voltage monitor
3. Output sockets for charging and tapping power to bread board.
4. An LED light for the bread board working
This tiny circuit can be hooked between the charger and the 9V or 12V battery. When the circuit is connected to the charger, LED remains off. But when the battery is connected, LED turns on. At first, the LED brilliantly lit indicating that current is flowing to the battery. Once the battery is fully charged LED turns dim.
There is a resistor R2 ( R Sense ) between the Emitter and the base of the PNP transistor. Its value is selected using the formula
0.65 / I in Amps
So we need a specific current drop across the R Sense resistor. Let us fix it as 24 mA to control the working of T1.
0.65 / 0.024 A = 27 Ohms
So when a current higher than 24 mA passes through the R Sense resistor, LED brightness increases.
R3 is the current limiting resistor for charging. So along with R2, it gives 127 Ohms resistance. So the charging current will be
12V / 124 = 0.094 Amps or 94 mA. It is good for slow charging. But this is the theoretical value. You can expect a current between 80 mA and 100 mA due to tolerance of the resistors used.
Then make the Battery Monitor circuit
This simple Battery Monitor can be hooked to the 9 V battery so that it indicates the voltage in the battery through visual indications. When the Test switch is pressed, the Bicolour LED lights as Green if the battery voltage is above 9V (fully charged) , it lights Yellow when the voltage is between 7.5V and 9V, ( moderate charge ) and lights Red when the battery voltage is below 7.5 ( needs charging )
Two Zener diodes (ZD1 and ZD2) works like switches. When the battery voltage is above 9 volts (A fully charged 9V battery shows 9.2 – 9.5 terminal Voltage), Zener diode ZD1 conducts and give full base current to T1. T1 then conducts pulling its collector voltage to ground potential. So the Red half of the Bicolour LED turns off. Green LED still lights since ZD2 is conducting. This gives Green colour to LED to indicate that the battery is fully charged. When the battery voltage goes below 7.5V, ZD1 turns off. T1 also turns off at this time. So Red LED turns on with full brightness than Green LED and due to visual perception, the LED looks like red. But between 7.5 and 9 V, there is partial lighting of both Red and Green halves giving Yellow colour to LED.
Then make the connectors