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The eLabtronics Pulser, Part 1

Pulse horns, lights or solenoids

by Julian Edgar

Click on pics to view larger images

At a glance...

  • Fully built cheap electronic module
  • Full user adjustment of pulsing characteristics
  • Lets you:
  • ...flash a shift-light
  • ...pulse a horn
  • ...flash headlights
  • ...pulse an intercooler water spray
  • Switch with changing temp or voltage
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As the name suggests, the Pulser module is designed to pulse the output. Whenever something needs to be turned on and off on a regular basis, the Pulser can probably do it. And that’s whether it’s pulsing a horn as an alarm, switching on an intercooler water spray for 5 seconds every minute, or flashing a warning light. You can even automatically flash your headlights – perfect for alarm or show uses.

The Pulser is fully built – for most applications, all you need to do is to make only four wiring connections and put it in a box. The Pulser costs just AUD$59: eLabtronics Pulser (Pre-Built Kit)

Using It

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The Pulser is based on the eLabtronics Multi-Purpose Module (see The eLabtronics Performance Modules). It has a high current output transistor called a MOSFET, a fuse, four wiring connections, an option switch (not used in Pulser configuration) and two user-adjustable multi-turn pots.

Click for larger image

Let’s look at the functions of the pots first. As can be seen in this picture, one pot controls frequency and the other pot, duty cycle. So what do these terms mean, and how do you use the pots to achieve the effect you want?

  • Frequency Pot

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If you think of anything that pulses on and off, there are two different factors that can be varied. The first is frequency – how many times per second it turns on.

For example, a flashing LED shiftlight might flash 3 times a second – that’s pretty fast. On the other hand, once triggered, an intercooler water spray might turn on once every 15 seconds – that’s clearly much slower.

By turning the frequency pot, the pulsing rate can be varied from 10 times a second right through to once per hour. That’s a huge range. Turning the pot clockwise increases the frequency (ie makes the output pulse faster).

Note that the action of this pot is non-linear, allowing the accurate setting of the frequency from very fast to very slow.

  • Duty Cycle Pot

The other aspect that can be varied is duty cycle.

Click for larger image

Think about that intercooler water spray that we described above as coming on once every 15 seconds. If it sprayed for half the time (ie 7.5 seconds) it would be said to have a 50 per cent duty cycle. If it sprayed for three-quarters of the available time (ie 11.3 seconds) it would be said to have a 75 per cent duty cycle.

Now the reason that you’d pulse an intercooler water spray is to save water (ie give time for the sprayed water to evaporate) and so a reasonable duty cycle to use might be 5 seconds every 15 seconds – that’s 33 per cent.

By turning the duty cycle pot, duty cycle can be adjusted from 0 per cent (ie the output is never on) right through to 100 per cent (ie output always on). Normally, of course, you wouldn’t have this pot set to either extreme.

Turning the pot clockwise decreases the duty cycle (ie makes the output pulse a smaller percentage).

  • Varying Frequency and Duty Cycle

One of the huge advantages of the eLabtronics Pulser is that frequency and duty cycle can be independently varied at will.

For example, if you want to flash a high power LED (say as a hazard flasher or even as an in-cabin ‘alarm on’ indicator) you’ll want to use as little battery power as possible. By setting the duty cycle very short (eg 20 per cent) and running at a high frequency (eg 3 or 4 times a second) you can create an attention-getting indicator that uses about 80 per cent less power than if the LED was on all the time. That’ll save your battery from going flat!

Click for larger image

Or perhaps you have a car with a water/air intercooling system. By setting the Pulser to switch on the pump for 15 seconds every minute, you can keep the water circulating (and so the intercooler heat exchanger cool) without wearing out the pump.

Note that in most applications there won’t be any ‘right’ frequency and duty cycle – you simply adjust the pots to alter the output to achieve what you want for the application. You don’t even need to know what the resulting frequency and duty cycle actually are – just adjust the pots until the system is working correctly.

Output Power

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The output MOSFET (transistor) is rated to handle a continuous 10 amps – but that’s when it is fitted with a big heatsink. How hot the MOSFET (and the circuit board) get depends not only on the output current but also the duty cycle. If the current is high but the duty cycle is short (eg 20 per cent) then the MOSFET will be able to cool down between each pulse. But if the duty cycle and current are both high, the device will get hot and need plenty of heatsinking.

For short pulses, the heatsinked MOSFET will handle up to 15 amps.

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As a rule of thumb, no heatsink at all will be needed if you’re operating warning lights, LED shift lights or beepers.

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If you are pulsing a string of low power filament lamps, a small heatsink will be needed.

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If you’re pulsing a pump, a medium sized heatsink will usually be needed.

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Finally, if you’re pulsing multiple car horns or multiple headlights, a large heatsink will be needed. Remember, in each case, the longer the ‘on’ time of the pulse, the greater the need for a heatsink.

The heatsink needs to be isolated from ground and positive supplies, so either mount it so it fits inside a box (and can’t touch anything metallic!) or mount the heatsink to the MOSFET using an insulating spacer and nylon nut and bolt. In either case a smear of heatsink compound will be needed between the MOSFET and the heatsink.

Don’t forget that in most uses of the Pulser, no heatsink – or only a small heatsink – will be needed.

Ultra High Currents

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But what if you want to operate really big electrical loads – like multiple radiator fans, high-powered sirens or the like? There’s no problem – you’ll just need to buy a solid state DC relay. These relays are fully electronic, so have no moving parts.

In addition to being very durable, an electronic relay can switch very large currents. When equipped with a suitable heatsink, the relay shown here can handle 100 amps continuously and cope with a very short term switch-on current gulp of 240 amps.

When using an external sold state relay, the Pulser MOSFET doesn’t need to use a heatsink, so packaging becomes easier – the Pulser can easily fit into a box and the solid state relay can be mounted remotely.

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This diagram shows how the relay is wired to the Pulser module. The electronic relay is available from the AutoSpeed shop for AUD$40 – see Solid State Relay.


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The eLabtronics Pulser has just four connections. These are:

  • Power - marked on the board as ‘+’. Power is nominally 12V and you’d usually use an ignition-switched source.

  • Ground – marked as ‘-‘. You’d normally connect this to chassis earth or the negative terminal of the battery.

  • Input – marked as ‘in’.

  • Output – marked as ‘out’.

Let’s take a look at the ‘input’ and ‘output’ terminals in more detail. (Click on the pics to enlarge.)

Click for larger image

When the Pulser’s output MOSFET is turned on, battery power is available at the output terminal. So all you need to do is to wire your load (lights, buzzers, horns, solenoid, etc) between the output terminal and chassis ground. If the load has a polarity, the positive terminal goes to the Pulser. (Note that as with all MOSFETs, there is a slight voltage drop across it, so at high loads, a little less than full battery voltage will be available at the output at high loads.)

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To switch the Pulser on, the input wire needs to be connected to 12V. Therefore, at its simplest, you just connect the input wire to the power supply – putting a switch in that wire to turn the Pulser on and off.

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The input wire takes only a tiny current, so even if you have the Pulser operating something that’s pretty current-hungry (like a horn or pump), the input wire switch can be rated for nearly no current. This function is really good because it means the Pulser acts like a high power electronic relay – you can switch the input using a low current micro-switch, pressure switch, etc.

In fact, the input wire of the Pulser turns on when it receives a voltage above about 2.6 volts. Therefore, it’s possible to use a sensor to switch on the Pulser when a signal rises above this voltage. We’ll have more on this in Part 2 of this series.

Setting Up

OK, so how do you set up and test the system? Firstly, place the module so that its underside tracks can’t short-out to ground. For example, if you’re working in a car, place the module on a seat or carpeted floor. Then complete this checklist:

  1. Power connected

  2. Ground connected

  3. Load connected between ‘out’ and ground

  4. Input connected (perhaps via a switch) to Power

Click for larger image

Frequency pot set half a turn anticlockwise from the fully clockwise position. (Note: These pots are multi-turn so don’t expect to make only one rotation when setting them. Multi-turn pots also don’t have clear end-stops [although they can sometimes be heard clicking when they’ve reached the end of their adjustment] ).

Duty cycle pot set to roughly the middle position.

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When the input is connected to power, the on-board red LED (arrowed) will light, showing that the module is triggered. The output pulse will also immediately start. (This is very useful because if you have the output set to pulse for 15 seconds every hour, you don’t want to wait an hour to see if the wiring is right!)

If nothing happens, check your wiring and then the module’s fuse. Make sure that you don’t have the frequency pot set too fast or the duty cycle pot set to either extreme end of its travel (ie 0 per cent or 100 per cent).

If all is working correctly, adjust the frequency and duty cycle pots (in that order) to gain the results you’re after.

With the settings finalised, make sure that the output MOSFET (or heatsink, if fitted) isn’t getting too hot – it’s OK if it grows very warm but it shouldn’t be too hot to touch. If it is hot, increase the size of the heatsink or add an external solid state relay (see 'Ultra High Currents' breakout box above).

If no heatsink (or only a small one) is fitted, the module will fit into this box.


The Pulser achieves a very simple aim – switching things on and off. The beauty of the design is in its compactness, power handling ability and the ease with which a wide range of pulsing behaviour can be attained. If in a car you need to switch anything on and off with a regular repeating pattern, the Pulser will be hard to beat.

Next week: using sensors to automatically trigger the Pulser

The Pulser is available fully built and tested from the AutoSpeed Shop - see eLabtronics Pulser (Pre-Built Kit) .

Pulser Specifications

Input voltage: 10 – 40 V DC

Output power: up to 10 amps continuous with appropriate heatsink, up to 15 amps short pulsed with appropriate heatsink, up to 100 amps with appropriately heatsinked external solid state relay

Wiring connections: power, ground, input, output

Frequency: adjustable from10 times a second to once per hour

Duty Cycle: adjustable from 0 – 100 per cent

Fuse: 15 amps

The eLabtronics modules are engineered and manufactured by eLabtronics. The modules are based on concepts and specifications developed by Julian Edgar, with the aim being to provide cost-effective and useful modules for car modification (and also industrial and educational uses).

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