I saw this youtube video ( Building The World's Loudest Alarm Clock - YouTube ) in which a guy makes an alarm clock trigger a car horn.
My question is since the PNP Transistor turns on when the current flows from Emitter to Base, how come that current flowing into the positive signal of the alarm clock does not damage the clock.
From the factory, the conventional current from the positive signal was meant to flow through the motor, and after this modification, current now flows from the transistor into that positive signal wire?
(please see attached circuit diagram)
5v on the base resistor of the PNP ‘will not’ turn off the transistor.
You must provide 0v to +12v levels going to the base resistor.
how come that current flowing into the positive signal of the alarm clock does not damage the clock.
It certainly could. The design is unworkable, so give that video a thumbs down.
The signal from the mysterious "alarm clock" is probably capacitively coupled.
I think I'd just go with that circuit from Larry's pic ----- at location 'E1' in the image.
The circuit in the opening post image won't work for 0 to 5 volt levels for the input - which was already pointed out (mentioned) in the thread.
I looked for simple errors that if corrected would explain the operation.
I can only come up with one - it could work if the alarm clock was an open or closed contact.
In which case the choice of base resistor is poor (ie ridiculous).
The alarm clock perhaps has "dry contacts"? (IE a reed-relay).
Or is open-collector output, so that any reasonable voltage will work.
Now granted being a child of the late 50s early 60s when it comes to car horns all of the horns I remember were a single wire device with the positive switched and the negative being bolted to the car chassis. Now if that is still the case doing low side switching on a car horn may get a little tricky unless the actual car horn ground is off the chassis. Don't know if horns on cars and light trucks use a relay either. Cars and light trucks have developed a love affair with relays.
Anyway if using low side switching just make sure whatever you have is compatible with low side switching.
Ron
The transistor turns on with any voltage < 12V-0.7V.
The alarm clock would have to have a signal high >11.3V and a signal low <11.3V for it to work.
Any 0V to V+ switching signal where V+<11.3V won't work. Of course any relay or reed switch
shorting the base resistor to GND works perfectly.
I used a function generator with a 5V p-p squarewave output feeding the V+ input of an op amp running off 12V with the V- pin tied to GND and the output connected to the base resistor.