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)
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
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?
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
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.
The other aspect that can be varied is duty cycle.
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).
One of the huge advantages of the eLabtronics
Pulser is that frequency and duty cycle can be independently varied at
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!
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.
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.
As a rule of thumb, no heatsink at all will be
needed if you’re operating warning lights, LED shift lights or beepers.
If you are pulsing a string of low power filament
lamps, a small heatsink will be needed.
If you’re pulsing a pump, a medium sized heatsink
will usually be needed.
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.
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
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.
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.
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.
The eLabtronics Pulser has just four connections.
Let’s take a look at the ‘input’ and ‘output’
terminals in more detail. (Click on the pics to enlarge.)
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.)
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.
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.
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
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:
Load connected between ‘out’ and ground
Input connected (perhaps via a switch) to
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
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
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
voltage: 10 – 40 V DC
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
connections: power, ground, input, output
Frequency: adjustable from10 times a second to once per hour
Cycle: adjustable from 0 – 100 per cent
Fuse: 15 amps
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).