Electronic_SCL_Std

Teletype Electronic

Simple Current Loop Interface with Standard Components

This example is more simple but fit all you need to communicate with a teletype without a FSG (dialing device). It is designed to use common components which should be found in every makers stock.

In this example the teletype current of 40mA is provided by a resistors in line (potentiometer) and have to be calibrated to 40mA. In stead of making he power supply yourself, you can use a (historic) teletype test device and put this circuit in line at any position of the loop.

The ESP part and the teletype part are fully isolated from each other.

If need a relay to power on the teletype motor, use relay transistor parts from schematic above.

Status: tested and working

Note: TXD have to be NOT inverted and RXD have to be inverted in software

Note: The bridge rectifier can be skipped if you are sure you do not change poles!

Components

Bridge Rectifier:

• any with Vrrm>100V and I>100mA

Opto coupler Receiver:

• PC817, LTV817 (Vce=35, CTR>50%)
• PC816, LTV816 (Vce=80, CTR>50%)
• any opto coupler with CTR>50%

Opto coupler Transmitter:

• PC817, LTV817 (Vce=35, CTR>50%)
• PC816, LTV816 (Vce=80, CTR>50%)
• any opto coupler with CTR>50% and Vce includes voltage source

  Note: The transmitter voltage tolerance (Vce) must be higher than the power supply voltage

Transistor Transmitter:

• MJE230 (Vce=300V, Ic=500mA)
• BD139 (Vce=80V, Ic=1.50A)
• any NPN transistor with hfe>25, Ic>200mA and Vce includes voltage source

  Note: The transistor voltage tolerance (Vce) must be higher than the power supply voltage

PCB Layout Example