# Electronic – 0-10 second on-time with a potentiometer

delaypotentiometertimer

I would like to make a simple circuit, where the a potentiometer controls a 0 – 10 second on-time.

If the potentiometer is set to min, the on time should be 0 seconds. If the potentiometer is maxed out the circuit should be on 10 seconds.

I was thinking of using an Arduino, but that would be overkill for such an application. And too expensive as well. Maybe a 555 timer would be an option?

All help is appreciated!

Notes:

• Circuit must be 5V or 12V DC
• The object to turn on is a 12V car lightbulb. (some advice about a relay?)
• Has to be cheaper than using an Arduino

The NE555 datasheet provides you with circuits in section "Application Information".

## Monostable operation

When you want the device to go on for 10s and then turn off, use the monostable operation: In the datasheets you can find graphs how the pulse width (your 10s) depends on \$R_A\$ and the capacitor on pin 6. For a 10s you can use \$R_A = 1\text{M}\Omega\$ and the capacitor \$10\mu{}\text{F}\$ (see figure 11 of the datasheet). Decreasing the resistance will shorten the pulse width, so with a 1M potentiometer you'll be all settled. \$R_L\$ is just a pull-up.

## Astable operation

When you want the device to toggle all the time, you can use the astable operation: The frequency (you want 0.1Hz - unlimited) can be adjusted with \$R_A + 2R_B\$ and the capacitor at pin 6, again. For a 0.1Hz operation you can use \$R_A + 2R_B = 1\text{M}\Omega\$ and a capacitance of \$10\mu\text{F}\$ (see figure 14 of the datasheet). Adjusting \$R_A + 2R_B\$ to zero will increase the frequency and thereby shorten the pulse width.

You of course will only make one resistor a potentiometer. The other has to be that small that when that's the total resistance (it's the minimum) the frequency is as high as you want it to be.

Again, \$R_L\$ is just a pull-up.

## Linking to the light bulb

The NE555 can run a relay directly, as is explained here:  As you can see, you can sink and source the current for the relay coil (thus you can choose if the relay should be on or off when the 555 is on or off). In both cases, you'll need a flyback diode to eliminate the sudden voltage spike over the coil when the relay switches.

Combining the monostable operation circuit with your needs and the relay circuit: 