Introduction: 1.5v LED Flasher Using an Old Clock

Hi.. This is my first instructable ! I had been tinkering with electronics since my childhood .. I am a doctor now but i still love building and exploring circuits and learning how stuff works..

LEDs dont light up in 1.5v usually if connected directly since the forward voltage drop is a bit higher .. The least being usually around 1.8v for RED leds..

This circuit is pretty efficient and can run on a single AA battery for about a year or even more!

Lets get started!

Step 1: Parts Required

U will need the following :

1. An old wallclock (not the digital one! )
2. NPN transistor (BC547 / 2N3904 / 2N2222 )
3. Resistors (1K , 150R/220R )
4. Red LED (green and yellow will work but wont be as bright and Blue wont work)
5. Capacitor (220uF / 330uF / 470uF)
6.Perf board
7.Soldering tools
8.some TIME ;)

Step 2: The Circuit

Get the circuitry out from the old wallclock and you will find that it has got a pad connecting to positive terminal of the battery and second one to the ground terminal and 2 more for the solenoid.

Remove the connections to the solenoid

Build the rest of the circuit on a breadboard or a perfboard

Note the polarity of the capacitor!

U can connect the base of the NPN transistor to either of the terminals that were connected to the solenoid.

Once you connect the battery , it should start flashing exactly once every 2 seconds !
There you have it ..
0.5 Hertz Quartz precision LED Flasher !

Step 3: Modifications

Perhaps you want that single LED to flash once every second.. No problem !
All u need is one more NPN transistor

1. Connect the collectors of both transistors together

2. Connect the emitters of both transistors together

3. Connect the base of one transistor to (A) and the base of second transistor to the other pad (B)

Now when u connect the battery, u will notice that your red LED flashes exactly once per second

Perhaps you want 2 LEDs to flash alternately (one LED every second) .
Thats easy! U need to build a separate circuit as in step 2 and connect the base of the second transistor to the other terminal (B)

Step 4: Working Principle

First we have to know how the clock circuitry works:

In a nutshell , the solenoid is energized in the opposite directions every cycle! Which means

At first point (A) is positive and point (B) is grounded

Exactly after a second the polarity is reversed.Now point (B) becomes positive and point (A) is Grounded

And the circuit works in such a way that the positive pulses alternate every second but the pulse lasts for a few milliseconds only

Now coming to our 0.5Hz flasher circuit :

1. Initially , the LED does not light up cuz it sees just 1.5 volts at its ends

2. The capacitor charges to the supply voltage through 470R and 1K resistors

3. The transistor is off since the base connected to either (A) or (B) is grounded at the moment

4. When the positive pulse occurs , it switches on the transistor momentarily and once the transistor is on, the positive terminal of the capacitor is pulled to the ground

5. The LED produces a brief Flash since it now sees 3volts approx (battery voltage + capacitor voltage) since the capacitor is now in series with the battery

6. After that brief pulse , the base is grounded and the transistor is switched off , the capacitor begins charging through the two resistors and the cycle repeats

Step 5: Conclusion

This circuit is extremely efficient!
I dont have an oscilloscope or a very good fluke multimeter to measure the exact current consumption.. But i did try using my cheap multimeter and during the pulse it draws about 1mA and between pulses the current draw drops to as low as 10uA before the next pulse happens..

The 0.5 Hertz flasher should easily last for more than a year on a single AA battery !
The current consumption will be a bit more for the dual led alternating flasher version and the single LED 1Hertz version

Red LED gives the best results with a bright flash ! And i have tried with a used AA battery and it was still flashing well at just 0.86volts !!