Adding a Delay to the Voltage Switch

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!