The range of electronic performance car kits developed by Silicon Chip
magazine allows you to do pretty well anything you want in the way of
automatically switching devices on and off. You can turn things on when speed or
voltage rises above a certain level (voltage and frequency switch kits), you can
have devices automatically operate devices when a module detects that you’re
driving hard (delta throttle timer kit), and you can use the timer kit to give
delays or extended ‘on’ times (auto timer kit).
But recently we had an application where two different requirements were
needed – a relay had to trip when a voltage exceeded a certain level, and then
it had to stay tripped for a certain length of time before switching off.
Easy, you’re saying? Just use the Simple Voltage Switch and Auto Timer kits?
That’s dead right, but if you’re pushed for space or on a very limited budget,
there’s another way of doing it. All it involves is changing a few components on
the Simple Voltage Switch board.
The New Components
To provide the delayed ‘on’ time, you’ll need these components:
- MTP305 mosfet
- 2.7 meg ohm resistor
- 4.7uF 16V capacitor
- 1N4004 diode
Jaycar Electronics (and other electronics components stores) sells these bits
– they’ll cost you just a few dollars.
The mosfet replaces the transistor that operated the relay. The mosfet is
used because it will stay on with only a very small voltage applied to it, so in
turn keeping the relay switched on. The capacitor and resistor set the length of
additional ‘on’ time, and the diode prevents reverse current flow that could
otherwise upset the timing operation.
Step by Step
So how is the Simple Voltage kit modified?
Build the Simple Voltage Switch kit and check that it works correctly.
Remove transistor Q1, the BC337. This is best done by using a solder sucker
to draw away the solder that holds it in place. Heat the solder joints and one
at a time, use the sucker to remove the solder. The transistor will then be able
to be pulled out.
Insert the MTP 305 mosfet where the BC337 once sat. Be careful that you place
it on the board in exactly the orientation shown here. You may need to drill out
the PCB holes to allow the legs of the mosfet to easily fit through the holes.
Once you have placed the mosfet on the board and soldered it into place, test
the kit and make sure it still works normally. If it doesn’t, you have probably
put the mosfet in the wrong way around. In that case, unsolder it and re-install
it correctly.
Unsolder and remove the resistor shown by the red arrow. Unsolder the upper
end of the green arrowed resistor and leave this end hanging in space.
Solder the 1N4004 diode onto the board so that the end with the band goes to
the free lead of the resistor and the other end goes into the PCB hole. In other
words, solder the diode in series with the resistor, band end to resistor.
Solder the 4.7uF capacitor into place where the removed resistor once sat,
negative of the capacitor towards the top of the PCB. Then, solder the 2.7
meg-ohm resistor across (ie in parallel with) the capacitor.
Test that the kit still works – it should turn on as it did previously but
once the signal input voltage drops below the trip-point, the relay should stay
on for another 15 seconds. If you want a longer delay, increase the value of the
capacitor. If you want less of a delay, reduce the size of the capacitor.
Versatility
With this modification in place, all the normal good points of the kit still
apply. That is, you can switch when the voltage is rising to the trip-point or
falling to the tri-point, you can set the trip-point very accurately, the input
won’t load down a signal so the voltage switch can work off voltage-outputting
engine management sensors, the hysteresis is adjustable, and the output is a
DPDT relay.
It’s just that now you can also have a delayed ‘on’ time!
Applications for the modified switch include:
- Keeping an intercooler spray running for a minimum time (the spray could be
triggered from the MAP sensor or airflow meter output voltage)
- Keeping a radiator cooling fan running for a period after engine
switch-off
- Keeping a warning light illuminated for a longer period after it has been
tripped
- Simple turbo timer (timing period triggered by the MAP sensor or airflow
meter output voltage)
Conclusion
It adds little to the price and takes only a few minutes to make the
modification but it opens up lots more ways of doing things!
Simple Voltage Switch kit
Simple Voltage Switch preassembled
Preventing Engine Shut-Off
Our application was very unusual – but as we’ve said in the main text, there
are plenty of mainstream uses for the modified kit.
We used the module to prevent the petrol engine in a modified turbo hybrid
petrol/electric Prius from switching itself off immediately after being on
boost. This could otherwise happen after climbing a steep hill – on the run down
the other side, the engine could stop, potentially starving the still-hot turbo
of oil.
But how did the modified Simple Voltage Switch prevent that happening? Rewind
a few steps. This model Prius (it’s an NHW10) has two air conditioning modes. In
High mode, the engine is forced to run continuously. In Normal mode, the engine
is allowed to switch off whenever the hybrid ECU figures it should be off. When
High mode is selected, the air con system tells the hybrid ECU that it should
not switch off the engine by means of an ‘engine on’ request signal. This signal
is very simple – above 4 volts means keep the engine running, below 1 volts
means it’s OK to switch it off. So if we feed 5V to the ‘engine on’ input of the
hybrid ECU, the engine will keep on running.
Hmmm, OK. Now, what if we use the Simple Voltage Switch (SVS) to monitor the
airflow meter signal, sensing when engine load is above a certain threshold -
one that is set to correspond to a few psi of boost? That way, the SVS’s relay
will click over whenever load is high. By using a voltage divider (a pot) across
the regulated 8V supply used in the SVS, we can then feed the required 5V signal
to the hybrid ECU whenever engine load is high.
Add the delay to the SVS, so that the relay stays on for 30 seconds after the
engine load drops below the threshold, and the engine will stay running for that
length of time after a boost event, even if the hybrid ECU says it’s OK for the
engine to switch off.
So how well does the system work? In most cases the delayed ‘on’ time for
engine running isn’t noticeable – when the car is moving, it’s hard to tell
whether the engine is running or not. But if the car is being punted hard around
the city, pulling-up at a red traffic light causes the engine to keep running
for a short time, when previously it would have switched off as soon as the car
was slowing for the stop.
Mission accomplished!
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