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transistor circuit question, and an IVA one too
blakep82 - 2/7/14 at 05:46 PM

first up, transistors
i know theres a few ways to do this (oil pressure and battery charge related, but i think this might be a neater solution, if not a little bit more complicated)

I've got one of the CBS start buttons with an LED in it, I want the LED to come on when the engine's not running, and go off when it starts.
I've got a rev gauge which consisted of 5 LEDs controlled by megajolt, just set to different RPMs, i want the first one to come on when the engine is at idle. the lowest value it can be set to is 500rpm, and i'd guess it should idle around 800 or so? so fine, when the engines running, the first light is on, lets call that led, led 1, and the starter button led 2.

I'm after a circuit that when led 1 is on, led 2 is off
and when 1 is off, 2 is on.

the user outputs can go to a current up to 500ma, so should be well within that
I've just got no idea how to spec transistors, or make the circuit up.


the second question about IVA is toggle switches. always said they can't really be used on the dashboard (except in the exampt area behind the steering wheel,

however: "Interior design features and fittings in the “specified zone” which are contactable with a 165mm sphere in a forward direction must have a radius of curvature of 2.5mm or greater, except where a projection from a panel is less than 3.2mm where the edges must be at least blunted."

so if i had a toggle switch with one of those rubber covers, making the end of it 5mm wide, then it should be ok?

I'm thinking i really should have switches for ignition and fuel pump, so i can put one or the other on individually. there will be times when i want the ignition on without the fuel pump pumping away. also, do i need a fuel pump relay? as far as i can tell the current draw is only about 1.5A

the other option is rocker switches, but i don't like the look of most of them, especially those ones you used to get in minis.

theprisioner - 2/7/14 at 05:54 PM

It is called a bistable:

http://www.electronics-tutorials.ws/waveforms/bistable.html

snowy2 - 2/7/14 at 05:56 PM

If the toggle switches are behind the steering wheel they are ok I believe.

rallyingden - 3/7/14 at 09:39 AM

Just Wire the LED in the start button asan alternator waring lamp. That way when the engine ( alternator ) is running the light will go out.

And yes toggle switch covers will be OK for IVA if greater than 3mm radius.

RD

[Edited on 3/7/14 by rallyingden]

[Edited on 3/7/14 by rallyingden]

MikeRJ - 3/7/14 at 09:50 AM

quote:
Originally posted by theprisioner
It is called a bistable:

http://www.electronics-tutorials.ws/waveforms/bistable.html


A bistable is clocked between two different states, which is not what the OP needs. A simple inverter will do the job.

The Megajolt uses low side switches for it's outputs, i.e. the pins on the MJ unit get grounded to turn the LED on. If the start switch LED has an integral current limiting resistor (usually described as a 12v LED) then something like the following circuit is needed.

Megajolt LED inverter
Megajolt LED inverter

None of the components are very critical. The transistor and the diode D2 must be rated for your LED current. A 2N3904 and 1N4148 are rated at 200mA maximum which should be more than adequate for most LEDs and both are very cheap and easily found. Almost any general purpose NPN switching transistor will do the job however, and you could also use something like a 1N4001 for the diode.

D1/R2 are the LED and current limiting resistor within the start switch.

The reason for the diode D2 is that the outputs of the Megajolt come from a darlington driver array which can only pull down to around 1 volt or so. Without the extra diode the transistor would remain switched on since the base-emitter voltage is only around 0.6v.

Resistor R1 biases the transistor into conduction when the MJ output is off. This simply needs to provide sufficient current to ensure the NPN transistor is saturated, 4K7 will be adequate even for very low gain transistors and a 10k should be fine for most switching transistors with typical LED currents.

If the start switch has no integrated current limiting resistor, of if you can bypass the resistor then a much simpler scheme can be used:

Simple Megajolt LED inverter
Simple Megajolt LED inverter

In this case, with the MJ output switched off resistor R2 powers the LED. 590 Ohms gives about 20mA into a red LED with 13.8v, this can be adjusted as required. When the MJ output turns on, the current is diverted from the LED into the MJ's output switch and the LED goes out.

The Black Flash - 3/7/14 at 03:34 PM

Alternative to the alternator is to wire the start button to earth to the oil pressure switch, so it comes on when no pressure. Alternator idea sounds better though, now why didn't I think of that?

I've used switches for fuel pump and ignition, both running via relays. The fuel pump switch was after reading some of the fire horror stories on here. The start button is in series with both so it only comes on when both switches are on, though tbh I now think it's probably as well to have it always live. It's useful while testing things to be able to turn each thing on individually.

But mostly it makes you feel like a fighter pilot when turning everything on

MikeRJ - 3/7/14 at 04:48 PM

quote:
Originally posted by The Black Flash
Alternative to the alternator is to wire the start button to earth to the oil pressure switch, so it comes on when no pressure. Alternator idea sounds better though, now why didn't I think of that?


Both alternator warning and oil pressure are not directly measuring engine operation. Low oil pressure or a faulty sender or slipping fan belt would indicate the engine is not running when it actually is. Not really important in this application, but could be for others.

blakep82 - 3/7/14 at 07:29 PM

Thanks for all the info there Mike
It'll take me a while to get my head around it though
If I get the current draw for the leds (both '12v' could you build one in a little box for me?
In fact, its basically a not gate isnt it?


Just to clarify a point, on the oil pressure/alternator thing, I wanted this effect to be a little more clear cut and instant.
If the engine stalls, the oil pressure light stays off a while until the pressure drops. Charge lights can flicker at idle, but I want it a definite on and off as soon as the engine starts, and on as soon as the engine stalls, if I stall it...

[Edited on 3/7/14 by blakep82]

MikeRJ - 3/7/14 at 09:25 PM

quote:
Originally posted by blakep82
Thanks for all the info there Mike
It'll take me a while to get my head around it though
If I get the current draw for the leds (both '12v' could you build one in a little box for me?
In fact, its basically a not gate isnt it?


Yes, it's effectively a logic NOT gate, also called an inverter.

I've just realised that you want to actually keep the LED on the output of the MJ to indicate idle (reading too quickly again). That may slightly complicate things.

Are the 5 LED's in your 'rev gauge' all separate parts, or is it some kind of assembly with a bunch of wires hanging out that's not easy to modify?

blakep82 - 3/7/14 at 09:49 PM

its an aluminium plate, with 5 leds glued in, all separate, and 10 seperate wires out the back
this, in fact
Description
Description

worth pointing out too, that megajolt can power these user outputs in normal (off until over the rev or load limit) or inverted, so on until over the limit) so it can power either led, leaving the inverter to power the 'bigger' load if needed

each was to be wired to the user outputs on megajolt (4 of them) the 5th is to go to the shift light

first LED is blue and is to be set to come on at 500 rpm (i'd guess the engine should idle around 800rpm) and basically just indicated the engine is running, although that should be pretty obvious without a light telling you!

when the engines is not running, i'd like the start button to light, and as soon as its started, i'd like it to go off (as the blue rev led comes on).
yes the oil pressure light or alternator light would do it, and probably be a whole lot easier, but not as slick

if the start button can 'pulse' even better lol (semi serious there!)

jeffw - 4/7/14 at 05:36 AM

quote:
Originally posted by rallyingden
Just Wire the LED in the start button asan alternator waring lamp. That way when the engine ( alternator ) is running the light will go out.

And yes toggle switch covers will be OK for IVA if greater than 3mm radius.

RD

[Edited on 3/7/14 by rallyingden]

[Edited on 3/7/14 by rallyingden]


The LED is unlikely to to excite the alternator as it has insufficient resistance. You would need to have a resistor in the circuit to make the alternator switch on...