I have a Sonifex Net-log audio recording device. It can be configured to begin recording on remote input. From the manual:

The remote inputs and output can be driven by 5V logic. The inputs are active low. These can be set to momentary or latching action via the Remote Options screen.

The connection of the Net-Log has four such remote inputs and a common ground plus outputs (which I'm not going to use). In momentary configuration, pulling an input pin to ground causes recording to start, releasing causes recording to end.

Consider the following diagram:


simulate this circuit – Schematic created using CircuitLab

Pressing SW1 causes recording to begin.

I have designed a CMOS logic circuit with various gates to decide when to record. The final output stage is a 74HCT08N AND gate. Originally I had planned to connect the input via a transistor to drive a reed switch to close the input pin to ground but I'm wondering if I can do without the switch part?

My question is, can I achieve this using a simple transistor? I think this could be implemented as a simple transistor switch but I'm not sure as I don't think any current will flow as such?

Do you think this arrangement will work:


simulate this circuit


1 Answer 1


Since the Sonifex can be driven by 5 volt logic, you should be able to connect your logic output directly to the input lines, without the transistor. You can add the transistor if you need a logic inversion, and/or if it makes you "feel safer".

In any case, your logic ground must be connected to the Sonifex ground.

If there is any problem connecting the grounds, you should use a reed relay or opto-isolater to provide galvanic isolation.

  • \$\begingroup\$ I suppose the bit I need to add is that I am sending the 'input signal' over PSN 25-pair cable running from the studio to the machine room. I don't really want to put 5v down this line if I can help it since pairs are shared by other things. I think this is what they call wet and dry contacts? I can tie the grounds together but have to consider the risk of creating ground loops so perhaps a reed relay is the way to go. I am interested to know whether what I propose above would work though \$\endgroup\$ Commented Mar 30, 2015 at 1:57
  • \$\begingroup\$ Relay or optocoupler (basically a photo transistor) will work just fine. \$\endgroup\$
    – Passerby
    Commented Mar 30, 2015 at 3:39

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