Nuts and Volts - January 2016

same way. To prevent destroying either or both phones, use an optical isolator to prevent the current in one phone circuit from affecting the other phone circuit (it also prevents ground loops which can cause interference and will reduce noise transferred between the two systems). Figure 7 shows an optical isolator circuit for connecting the VoIP phone line to the four-line phone's unused line.

The four input diodes insure correct operation of the circuit regardless of the phone line polarity (my residential line always shows as reversed on my store bought phone line tester). The opto-isolator prevents current from flowing between the VoIP phone system and the four-line system; this prevents damage to components in these circuits. If you do not want the unused four-line indicator to flash, eliminate the flash oscillator and tie both AND gate inputs to the opto-isolator output.

If your four-line indicator will operate from the five volt/11 mA TTL output from the AND gate, you can leave out the op-amp (you could do this with your VoIP output but there would be NO isolation and no circuit protection). The op-amp section will require some calculation based on the output you need from the isolation circuit to drive the four-line indicator. A value of 1 KΩ for Rin is a good starting point; calculate Rf based on the needed output voltage from Vout/Vin = (1+Rf/Rin). Build the circuit first on a breadboard in case you need to modify (add and/or delete modules) anything before you are ready to commit to a printed circuit board (PCB).

While writing this column, I experienced problems with my residential phone in which after a rain storm, the phone sounded like something was frying, our Internet signal was lost, and our phone/answering machine alternated between "Line in Use" (a.k.a., phone off-hook) and saying, "At the tone, leave a message." Our telco charges a large fee if their technician comes out and finds a problem with the internal residential wiring, so I checked

12 January 2016

■ FIGURE 6.

Motor Control Motor Control

• 15 Amps Per Channel • Dual Channel • Quadrature Encoders • DC Brushed Motors • USB / RC / Serial • 45 Amps Per Channel • Dual Channel • Quadrature Encoders • DC Brushed Motors • USB / RC / Serial