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I have theoretical understanding of general electronics basics, however I do not have any practical experience in circuit design. Therefore I need some advice of what would be the best practice on how to set up a circuit that has some requirements.

I have multiple industrial cameras that are triggered by one master camera. The master camera has an internal mosfet and the slave cameras are triggered by an internal optocoupler. Furthermore the trigger event should also go into a MARK_IN pin of a gps receiver which has an 3.3V high and 0.0V low level and registers the triggering event on a falling edge.

I set up a basic circuit in a simulation that I can think of that is shown below:

enter image description here

Here the 820 ohm resistors with the diode (led) are the internal trigger_in optocouplers of the slave cameras. The mosfet is the trigger_out when an image is taken by the master camera. Furthermore I thought about connecting the MARK_IN pins of the gps receiver like shown (I do not know how the internals of the MARK_IN pin looks like, datasheet only say it should be 3.3V with falling edge).

However, with my design I see a lot of problems:

  1. The voltage level does change at MARK_IN when the number of connected slave cameras changes. For example just one slave instead of 2.
  2. The energy consumption is about 150mW when the mosfet pulls the line on ground, which I think is quite a lot.

So I am wondering if there is a typical approach how to realize such circuit? That can handle a varying amount of slave cameras and still keeps the MARK_IN pin on 3.3V level. Am I on a right track or completly wrong? And furthermore, a circuit that is more energy efficient.

Apart from that is there anything wrong with my circuit in general, or something that I am missing?

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This will be easier if you add reference designators to all components.

2a) 150 mW (or 167 mW) is not a lot, although it would help to know the part number or at least the package type of the internal MOSFET to determine its power handling abilities.

1a) Look into an emitter follower circuit. This will give a more stable output voltage. Example:

  1. Increase the 150 ohm resistor to 1K (to reduce the MOSFET power dissipation.

  2. Add a small NPN transistor to the circuit, 2N4401, 3904, 2222, etc. Base to the bottom of the 1K resistor, collector to the +5 V, emitter is now the new MARK_IN.

  3. Increase the 1 K resistor on the MARK_IN line to 10 K. This should be low enough to assure a logic 0 at the GPS input. If not, return it to 1.0 K.

  4. Add a 3.9 K resistor from the transistor base to GND. With the 1.0 K, this forms a voltage divider that puts approx. 4 V on the base when the camera output is high.

After the transistor base-emitter junction voltage drop, the emitter (the new MARK_IN source) should be at around 3.4 V. Because of the current gain of the transistor, the MARK-IN line will be much less susceptible to changes in load.

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  • \$\begingroup\$ Thank you for the very detailed answer. Is this also possible to do it with a an IC? Or does an IC exist for handling such problems? \$\endgroup\$ Oct 10 '20 at 19:33

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