Electronic – Is this transistor switch circuit designed correctly

circuit analysispower supplyresistorsswitchestransistors

I am making a little home made electronics project.

The plan is to turn on a Red LED when a SCART input is sending an RGB signal. Pin 16 on a SCART cable is 1-3V when the input is carrying an RGB signal (according to wikipedia – in my case it actually sends 4.5V so I figure this is quite a flexible spec).

The circuit I have designed works….but I don't know if it is safe, or done correctly! Please see the schematic below:


simulate this circuit – Schematic created using CircuitLab

I have 2 main questions:

  1. I don't seem to need the 1k Ohm resistor leading to the base of the transistor. I assume any voltage applied to the 2N3904 base will switch the circuit on…so this is just keeping a low current to prolong the life of the transistor?
  2. The circuit doesn't work unless I cannot Pin 21 of the SCART input up to the ground of the main circuit. I don't know if this is safe, or the correct way to make this circuit work, can you validate my decision here.

Best Answer

1) The 1k ohm resistor IS necessary. The base - emitter junction of a transistor is a diode, and the voltage drop of a silicon diode is ~0.7 V. The transistor will allow as much current to pass as it need to to clamp that voltage to less than 0.7 V. This will interfere with the signal, as the RGB signal is indicated to the receiving device as a signal greater than 1V (1-3 V according to wikipedia: https://en.wikipedia.org/wiki/SCART). In order to keep interference to a minimum, I would probably use an analog buffer (https://en.wikipedia.org/wiki/Buffer_amplifier#Voltage_buffer_examples) instead of a transistor to turn on the LED, but if you find it doesn't interfere, there isn't much reason to change it.

2) If you find that pin 21 works as a ground for you, it shouldn't cause any damage. Looking at the pinout on that same wikipedia page, I would go with pin 18 instead. Pin 18 is specifically a ground for the signal on pin 16. Either way, a common ground is necessary for the signal to be able to turn on the transistor.