Here are two opto circuit I've been playing with for HAMCOM/JVFAX. They virtually eliminate all computer induced rfi. I've been using a five foot serial cable and twenty foot audio cable with this circuit and the results are truly remarkable with JVFAX. I've used the first circuit successfully, but recommend using the second. The first can be built with common Radio Shack parts. The second requires a little more uncommon LM358 and TIL181. The Radio Shack opto RECEIVER is only rated at 20v but used at 24v and could eventually burn out. It also may introduced problems since the +9v is not regulated and induces error into the reference op amp output ... even though it seems to work well. Perhaps some engineers out there may have some advice. =============================================================================== +9v |-------- +9V ________ DTR 9 volt batt. --------- | 1uf | | ----- === | | | | [ ] 1K [ ] 2.2K |-------- | |________DSR GND |C | +9v ___ |/ - C | \ / /\/ | .1 uF |\ | ----- |\-> A >------| |--*-----------------------|- \ |A | | |741 \______| | | | / --------RTS | --------|+ / | | |/ | [ ] 100K [ ] 100K | | | GND *--------------* | | +9V | |\ | | -----|- \ | | |741 \___| +4.5 REFERENCE 100K [ ] | / |-----------|+ / 100K [ ] |/ | | GND =============================================================================== INFARED EMITTER INFARED DETECTOR (clear) (dark) ---------- ---------- | | | | | |__ | | | |__ | | | | | | | | | | | -- | | | -- | | | | | | | | | | ---------- ---------- | | | | | | These are available at Radio Shack as a pair for $1.99 (#276-142). This shows which is the Anode and which is the Cathode, not clear on the package. The LED and DETECTOR must be positioned right up against each other and exactly aligned to get maximum light transfer. The sides with the little lense like dots must be placed together. =============================================================================== I'd recommend this second circuit for more stability and possibly accuracy. RS232 connector size ----> 25 9 +9v -- -- | o +9V ----o DTR 20 / SPST SWITCH | | o SW1 | | | R1 [ ] 1K R2 [ ] 2.2K 9V *-------- | | BATTERY | | | |----o DSR 6 --------- --- C1 |1 TIL181 |5 ----- --- 1UF ....................... | | . |C U2 |C . * ------- . ___ |/ - . | . \ / /\/ | . GND . ----- |\-> . +9v . |A |A . | ....................... AUDIO 100 nF 6|\ |8 |2 |4 o------| |--*------------|- \ | | | | U1 \_7____| ----o RTS 4 | 5| / | ----|+ / LM358 2|\ | | |/ |4 ---|- \ R3 [ ] R4 [ ] | | | U1 \_1___ (not used) GND 100K | 100K| | | 3| / o-----------*-------*--------*------------*---|+ / LM358 | |/ GND =============================================================================== | ** NOTE** | | After constructing, measure LM358 pin 7 with no audio on the input. | | It should be high (~8v), if it is low (~0v) it will drain the battery | | in only 20 hours or less. | | | | To correct this: Disconnect R1 from +9v and reconnect to U1 pin 7. | | Disconnect U1 pin 2 and reconnect to GND. | | | =============================================================================== The LM358 is a single supply opamp and can be substituted with the LM2904, LM158, or LM258. (The dual supply 741 can not be used as a substitute.) The opto isolator IC can be substitued for many other types. The 4N35 has been tested successfully. The discrete infared emitter/detector from Radio Shack has been tested successfully. I tried to use a darlington opto (TIL113), but couldn't get it to work. A darlington opto circuit would only require 1.5ma average. That would make battery life about 133 hours. Maybe I'll try again later. =============================================================================== MORE NOTES: The 9v battery could be substitued with a 9 or 12 vdc adapter. However I've found the clearest pictures occur when both the radio and HAMCOM are running off batteries ... at 6am 8). A 9v battery should last about 50 hours while audio is present (assuming 200ma hours for a battery). If no audio is present the battery will last 400 hours. The battery life could be a problem for people who leave HAMCOM running all night using the timed FAX option. Battery would only last a week. If you have any advice or knowledge that could improve this, please let me know. copeland@metronet.com