Serial switch via opto coupler/isolator

Hi all,

I am trying to build an interface to connect a transceiver to my pc in order to experiment with some data modes over the radio. I will use the sound card to produce the signals but intend to use the serial port to control the PTT (Push to talk). I have found an example of a circuit for this and it says to use the 4N32 opto coupler in order to protect the radio from the pc (and presumably vice versa).

**broken link removed**

What I wanted to know is whether this circuit would form the switch that I need. Also, having looked to buy the components there are lots of opto couplers/isolators around and I dont know why this one is better than others. This one seems to have a second transistor whereas the others have single ones, what difference does this make?

I am fairly inexperienced with actually using semiconductors in circuits as I have always stuck with relays that I could understand.

Thank you for any light you can shed on this!

Max

The main features on an opto isolator are:-

1) Isolation voltage. That is the voltage between the input and the output that opto isolator can stand without breaking down
2) Transfer ratio. That is really the gain, or how much more (or possibly less) current the output can take compared to the input current
3) Maximum output current.
4) Maximum output voltage
5) Turn on and turn off times.
6) Cost.

The second transistor probably improves the transfer ratio at the expense of speed.

I would have thought that in this application, just about any opto isolator would do, so item 6 is the most important.

You don't say what current the PTT switch needs to take. If that is more than 100 mA or so, it will limit you isolator choice.

What I wanted to know is whether this circuit would form the switch that I need. Also, having looked to buy the components there are lots of opto couplers/isolators around and I dont know why this one is better than others. This one seems to have a second transistor whereas the others have single ones, what difference does this make?

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The opto coupler shown is a "Darlington" type and thus the two transistors. Simply put it will handle more current than a single transistor version. The coupler makes a switch to ground as shown.

Something you may want to consider:

Voltage levels

The RS-232 standard defines the voltage levels that correspond to logical one and logical zero levels for the data transmission and the control signal lines. Valid signals are plus or minus 3 to 15 volts; the ±3 V range near zero volts is not a valid RS-232 level. The standard specifies a maximum open-circuit voltage of 25 volts: signal levels of ±5 V, ±10 V, ±12 V, and ±15 V are all commonly seen depending on the power supplies available within a device. RS-232 drivers and receivers must be able to withstand indefinite short circuit to ground or to any voltage level up to ±25 volts. The slew rate, or how fast the signal changes between levels, is also controlled.

Click to expand...

You may want to check the voltage levels for your DTR and RTS signals as to what a Logic 0 and Logic 1 actually are. That will help determine the value of the resistor on the LED (Diode) side of the coupler.

<EDIT> Beat by Diver,,, </EDIT>

Ron

Thanks

Thank you both for your replies! I will have to assume that the design includes the Darlington type as the current is higher than a single transistor can take. I am not sure what the current is, and I suspect it will change between radios, so I should probably go for a higher maximum to allow me to use it on more systems.

Thank you for bringing up the value of the resistor, I hadn't considered it and it was bound to catch me later. Am I aiming for a voltage across the LED of about 2V? In which case should I just make a potential divider using the specification from the optocoupler?

Thanks,

Max

The diode D1 can likely be a 1N4148 or similar. I saw a dozen PTT circuits all using a single transistor opto coupler (it figures). If you can give a part number for the opto-coupler I can give you some ideas for the resistor. For the diode side of the coupler the V forward and I forward need to be known. Then the logic 1 voltage out of the RS232 serial port needs to be known. If you need some software to test the port and measure the voltage let me know.

Ron

The forward voltage of the LED in the opto isolator will be about 1 V as it is an infra red LED. LEDs should be driven with a known current, rather than a known voltage.

The RTS or DTR lines will be at least 5 V and probably nearer 10 V. In fact, with the uncertainty of those, the LED voltage can be ignored.

The resistor and the RTS or DTR voltage set the current through the LED. If you are looking for 10 mA, then a 1 kΩ resistor will do.