# 0-10 second on-time with a potentiometer

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
• Do you want it to be on for 10 seconds and then turn off till the end of time, or do you want the device to toggle: 10s on, 10s off, 10s on, 10s off, ...? – Keelan Feb 4 '13 at 7:31
• No toggle, just 1 time constant 10 seconds. – nlstitch Feb 4 '13 at 8:34

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:

• Thank you for explaining the datasheet. - Could you give me a partslist according to the monostable operation? - Does the reset pin need to be on all the time? - To begin the time cycle, do I need to put 5V on de TRIG pin? – nlstitch Feb 4 '13 at 8:12
• @CamilStaps when I was your age (boy does that make me feel old), an NE555 would easily source/sink 200mA which should be sufficient to drive a small relay. Now I haven't been using 555 timers for ages and there are countless variations out there today, but an external transistor is not necessary depending on 555 device selection. The flyback diode is required though. – jippie Feb 4 '13 at 8:14
• It is still not clear to me what the circuit would look like. Could you make a simple and fast drawing? – nlstitch Feb 4 '13 at 8:29
• It's the top circuit in the question. – jippie Feb 4 '13 at 8:51
• @Jippie you're right, I updated my answer. See the updated answer for a customized circuit. – Keelan Feb 4 '13 at 9:21

I'd probably use a PIC 10F220. That has a 8 bit A/D, so you can connect the pot directly to the A/D input pin. It can look at the pot value and do the timing based on that. The result is a lot less parts and complications compared to a evil 666 timer. The only parts would be the PIC, bypass cap, and the pot. The power supply and whatever the digital output drives would be the same as with the ye olde phashioned analog timer.

yes, ON and OFF time can be easily set using the 555 IC. 555 is the great component in electronics, it is used to trigger most of the devices. We can use it in Monostable mode to set timer. Time can be set by changing value of resistor R1 (between PIN 8 and 7) and capacitor C1 (between PIN 6 and ground). You can use potentiometer in place of resistor R1 to change the time on board itself. Here is one example of time triggering: 1 minute Timer using 555 IC

As far as Relay is concern, you may find some circuit on above site to trigger the Relay. Although its easy to trigger Relay using 555 IC, and control the AC appliances.

ne555 is certainly an option. With capacitor of 10uF and resistor of 470K you can provide 5 seconds interval. So putting potentiometer in that range you can trim time duration value as you prefer. You can simulate ne555 monostable multivibrator to see all relevant waveforms.

http://www.cirvirlab.com/simulation/ne555_monostable_multivibrator_online.php

• Welcome to stackexchange! This answer would be better if you included a schematic. Could you do that? – Keelan May 7 '13 at 11:47