The Auto Timer

It seems like you’re faking an emotion when you get excited by a timer circuit. I mean, it times a period – so what? But the Auto Timer is a quite amazing timer – it can perform pretty well any timing function on a car that you can think of. It can pulse devices on and off, automatically switch off devices after a certain time, and it can be triggered either by having its input either connected to, or disconnected from, 12V.

And it gets even better. (Surely that’s not possible? – well it is!) You can set the timing period with great accuracy to be anything from one-tenth of a second to 16 minutes, 30 seconds. The timer also has an on-board relay so you can drive loads directly, and best of all, as a kit it costs only AUD$40. (It’s also available built and tested for extra cost.)

So what are its features?

Time Periods

The length of time that the timer operates for is set on two multi-position switches (arrowed). The top switch shows the time in units and the bottom switch in tens. So, set the top switch to 5 and the bottom switch to 1 and the timer is set to run 15 seconds.

However, this can be altered by the position of a moveable link. With the ‘multiplier’ link in the top position, this time setting is multiplied by 0.1, that is, the 15 seconds becomes 1.5 seconds. Move the multiplier link to the bottom position and the time is multiplied by 10, becoming in this instance 150 seconds. Leave the multiplier link off and the time is as shown on the multi-position switches.

One Shot or Pulsing

In many cases you don’t want a timer that just runs for the set period and then stops. Instead, you want the timer to be constantly pulsing on and off. In the case of the Auto Timer that’s easy – you just move Link 1 from its ‘1-shot’ to its ‘alternating’ position. The time set on the multi-position switches and the multiplier link still applies, but now the timer turns on and off at that rate. So, set the upper switch to 1, the lower switch to 0, and the multiplier link to open and the timer will switch on and off at 1 second intervals. Set the upper multi-position switch to 5 and the timer will turn on and off at 5 second intervals. Easy, huh?

Triggering

To add to its versatility, the timer can be triggered in two ways. With Link 2 set to its upper position (marked L/H or just L), the timer will be triggered if the input is connected to 12V. (That is, the input gets dragged up from a low voltage to a high voltage – gedddit?) However, sometimes you want a timer to trigger when the input gets disconnected from 12V. To achieve that, move the link to its H/L (or H) position.

Alternating Mode

In most applications, the options covered so far will give you all that you want. But designer John Clarke has built another very tricky feature into the Auto Timer. What if you want to pulse something on and off, but you want the on and off times to be different? In normal Pulsing mode, if you set the timer to (say) 3 seconds, the on and off times will each be 3 seconds – that is, on for 3 seconds, off for 3 seconds, on for 3 seconds, etc. But in some cases it’d be good to have a very short ‘on’ time. For example, if you’re pulsing the car horn as an alarm indication, 1 second beeps every 3 seconds is better than 1 second on, 1 second off.

It’s possible to put the Auto Timer into this Alternating Mode. It’s done by setting the timer to Pulsing Mode, then the top multi-position switch to 7. Then, with the timer powered up, two test points are briefly connected together. Once you’ve done that, you’re in Alternating Mode.

In Alternating Mode the bottom multi-position switch sets the ‘off’ time and the top multi-position switch sets the ‘on’ time. You can also use the multiplier link to change these times So for example, with the top switch set to 5, the bottom switch set to 2 and the multiplier link set to ‘x0.1’, the timer pulses on for 0.2 seconds every half a second.

Of course, with all these settings it’s easy to see how the timer is behaving. Not only can you hear the relay clicking on and off but a LED on the board lights each time the relay is switched on. Testing the timer on the bench is as easy as connecting 12V and earth and having a signal input wire that you can touch to the 12V input.

Overview of Uses

Nearly all Auto Timer uses can be categorised into four functions. These are:

• Time limiting – this means the Auto Timer is used to switch off something that would otherwise stay on until it was manually turned off or the battery went flat. An example is time-limiting the length of time a boot light will stay on with the boot open.

• Pushbutton timed period – this is the sort of use where you manually start the timed period, eg by pressing a button. An example of this is when you want to keep your headlights on for (say) 2 minutes while you make your way inside after coming home at night.

• Ignition-off timed delay – this lets you keep operating certain electrical functions for a period after you’ve switched off the ignition. Examples include power windows and the car sound system. You can also use it to automatically run a turbo cooling fan for 5 minutes after you switch off the car.

• Pulsing – this one we’ve pretty well covered in the text above.

So as you can see, this timer does it all. Next we’ll use it to pulse an intercooler water spray, an approach that saves a heap of water but has the same measured cooling affect as running the spray continuously.

The Background Story

So how did this kit come about?

The Auto Timer was developed and designed by Silicon Chip [www.siliconchip.com.au] electronics magazine. The kit, along with many others, is covered in the Silicon Chip publication, High Performance Electronics for Cars. The book is a must-have for DIY modifiers. The kit for the Auto Timer is available from Jaycar Electronics [www.jaycar.com.au] .

The electronics design and development of the Auto Timer was carried out by electronics engineer John Clarke, while I came up with the concept and did all the on-car development. (During this period I wore a different hat to an AutoSpeed contributor, working for Silicon Chip Publications as a freelance contributor to High Performance Electronics for Cars.)

So by no means should the Auto Timer be seen as an AutoSpeed-developed project, but at the same time I am happy that AutoSpeed endorses it and promotes it.

Julian Edgar

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