Switch on 3-5v circuit via 15v serial RTS

[nedge2k]

Hi, sorry if this is a stupidly basic question but i'd like to be able to switch on a 3-5v circuit using the ~15v RTS signal of a serial device. I gather I can use an NPN transistor for this but i'm a little confused about the resistors i may or may not need.

The serial device is quite expensive and i've no idea how much current can be drawn from it without damage so i'm hoping only a nominal current will suffice to switch the 3-5v circuit on. The current drawn by the 3-5v circuit should be no more than 35mA and the power source will be between 600-1150mAh.

can anyone suggest which transistor and resistors i would need?

 

[MikeMl]

Let' see if I understand what you want to do. When the RTS signal is asserted (~+15V), you want to switch on a 3 to 5V circuit which draws 35mA.

Is it ok to connect the minus side of the 3 to 5V circuit and its supply to the serial port ground, or should the two systems be isolated such as would be the case if we used an opto-isolator?

Presumably you want to switch the positive lead between the 3 to 5V supply and the circuit it powers?

 

[nedge2k]

got it in one. they can and currently do share the same ground.

originally i wanted to power the 3-5v circuit from the serial device itself but not knowing how much draw it could handle, i've opted for a external power source.

 

[MikeMl]

Wikipedia says:

"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 either in the range of +3 to +15 volts or the range -3 to -15 volts with respect to the ground/common pin; consequently, the range between -3 to +3 volts is not a valid RS-232 level. For data transmission lines (TxD, RxD and their secondary channel equivalents) logic one is defined as a negative voltage, the signal condition is called "mark." Logic zero is positive and the signal condition is termed "space." Control signals have the opposite polarity: the asserted or active state is positive voltage and the deasserted or inactive state is negative voltage. Examples of control lines include request to send (RTS), clear to send (CTS), data terminal ready (DTR), and data set ready (DSR).

This circuit does what you want. Will switch an external supply of 3 to 10V as is, higher voltages and power levels with some tweaking. Voltage drop when on is < 100mV. Base of Q1 is protected against negative voltage input by D1. Switching speeds not too bad; shows some effects of charge storage in the base of Q2. Will tolerate wide range of RS232 input voltages.

 

[nedge2k]

That's great, thank you!

I read something about this - darlington pair setup, correct?

Out of interest, could a voltage divider be used to bring the RTS voltage down and then the darlington method to up the current to power the circuit without risk of damaging the serial device?

 

[MikeMl]

That's great, thank you!

I read something about this - darlington pair setup, correct?

Out of interest, could a voltage divider be used to bring the RTS voltage down and then the darlington method to up the current to power the circuit without risk of damaging the serial device?

Not really a Darlington, more like a level-shifter.

The current drawn from the RTS pin by the circuit above is less than 1mA, which is well within what a Serial Port pin can supply. If you replace the NPN with an NFET, the current drawn from RTS could be tiny.

 

[nedge2k]

In that case, do you think something like this could work?

http://www.botkin.org/dale/rs232_interface.htm

Basically i'm communicating with and powering a TTL device.

 

[MikeMl]

That circuit is used to invert the logic sense of RXD and TXD; that is not what you asked for.

 

[nedge2k]

ah ok, thanks for the advice anyway - much more to go on now!