Transistor Switch Circuit - help please!

So, OK... you got me - this isn't strictly Arduino related!

However, I have recently bought a new car, and part of the tinkering I am undertaking is the following. I want to make the sidelights come on when the vehicle unlocks / when a door is opened. Fortunately there is already an interior light circuit to take a tap off. Unfortunately, the sidelight switch is ECU controlled and seems to use a 1.65V control line to sense the switch position. I have already tried shorting the red and grey wires (see below) and it produces the desired result - the sidelights come on, and no magic smoke escapes! Also on the downside, the interior lights are on a negative fader switch. I could realistically just pop in a relay to do the job, but due to slightly limited space and extremely low currents, I am trying to do it with a couple of TO-92 Transistors. However all the BJT's I have in stock have a 5Veb maximum, so trying to use a 12v range to switch them won't go to well.

Esentially - will the below (simplified) circuit become so bloated with protection components that a small relay would be a better plan?!

Many thanks,

Tom.

SIDELIGHT UNLOCK Trans.png|832x377

There must be a current limiting resistor in series with each base connection. Suggest 10k.

Note that the interior light dimming is performed by PWM.

I want to make the sidelights come on when the vehicle unlocks / when a door is opened. Fortunately there is already an interior light circuit to take a tap off.

A TO92 transistor is not going to handle the headlight current. Possibly a 2N2222 TO-18 package

Tap off the interior lamp bulb circuit to get your 12V . Use a 2.2k ohm series base resistor for the 2n2222. Connect transistor emitter to auto GND. Are the headlights connected to +12V on one side or to GND ? (Are the headlights being SOURCED from a relay contact or SUNK ? If the headlights had +12V already connected on one side ,you could SINK the current with the collector of the 2n2222. I don't know the headlight current so I don't know if a 2n2222 could even handle it . Probably not. In any case , a small 12V relay with the coil in parallel with the interior light and the N.O. contact used to connect the headlights to the COM relay contact. Again, not knowing how the headlights are wired I can't tell you how to connect the relay contacts.

The 5V rating for Vbe is the reverse voltage rating - ie for an NPN it means you must not take the base 5V below the emitter without risking destruction. (and vice-versa for PNP)

The forward rating for the base-emitter junction is usually a current rating, since the junction is forward biased it will be about 0.7V.

I think you meant VCEO (collector -emitter on)

FYI,
The 2n2222 TO-18 can handle 800 mA continuous collector current but I would strongly suggest you buy one of those little “hat” heatsink

Hi all, Thanks for the responses :)

Firstly, I think I have confused things - the current being switched, between the red and grey wires, is just an ECU signal. The ECU throws the relay for the sidelights separate to this switch, when the red wire is grounded. That the NPN (BC337-16 or 2n5088 are what I have lying around) will only sink 600uA at 1.65V from red to grey, so heat sinking or bigger packages aren't necessary.

Secondly, I meant Vbe; sorry! Can I just check: So if its a 5V Negative difference for the NPN, I'm fine to drive it 14V ABOVE the Emitter (Gnd), but I can't drop it to less than -5V? And conversely, I am fine to drive the PNP base down to Gnd, since its emitter is at 12V, but I can't drive it up to more than 17V without causing problems? (I have only got BC556B for the PNP)

Thanks folks.

It would help if you cite the source of your specs by posting a link to a datasheet with the spec label embedded in the link: (paste the URL in the FIRST box prompted and label in the SECOND box prompted) ie: Page-1/Maximum Ratings/Collector current-continous

Can I just check: So if its a 5V Negative difference for the NPN, I'm fine to drive it 14V ABOVE the Emitter (Gnd), but I can't drop it to less than -5V? And conversely, I am fine to drive the PNP base down to Gnd, since its emitter is at 12V, but I can't drive it up to more than 17V without causing problems? (I have only got BC556B for the PNP)

Sorry but my brain can't process that without something to visualize. I have no idea what you just said.

TomLS, your circuit works BUT Vbe can never be more than 0.7 volts, never, unless you let the smoke out. You MUST have a resistor from the interior light to the base on the PNP transistor. You also MUST have a resistor from the PNP collector to the NPN base.

For safe function also add resistors between base to emitter on both transistors. You can use 47 kOhm for all four resistors.

If the interior PWM get's the ECU box crasy, add an capacitor from the PNP collector to ground.

Pelle

A TO92 transistor is not going to handle the headlight current. Possibly a 2N2222 TO-18 package

Absolutely not! Car headlights are on the order of 10 to 20A.

Drive a relay.

That schematic is highly incomplete. Where are the headlights?

You need current limiting resistors on the base of bipolar junction transistors (BJTs).

Why not just put a diode in the existing cable to the relay, then add a second diode to the same relay. Wire this to your interior light. Job done.