Meteo in APRS with Arduino - project schematic question

[Lucky-Luka]

Hi all
I've found a project that should do what I've written in the subject of the thread.
Arduino is connected to a VHF Intek transceiver.
What do you think the circuit in the red circle do?
I've asked the guy who designed this project and he has told me that it's used since 90s with TNC packet radio.
I don't know... Is that diode necessary? Has that cap the right polarity?
Thanks

[pqass]

The circled part of the circuit appears to set the max. PTT on-time to about 10 seconds.  That is, if the MCU keeps the pin labeled "d9p" high for more than 10 seconds, the PTT output transistor will be turned off.

See simulation below.  I've changed the cap to 10uF to shorten the max. time to about 1 second.  If the MCU pin is  toggled for less than this max, then the PTT on-time follows the pin duty cycle.


http://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0l3AWAnC1b0DZwwExxwOwEDMkAHDhigSAKyR10O0CmAtGGAFAAu4OZEDhwIQZBsNEMIbBNCQ5aGImAUFahEVFgZixMkhI4w+4ZFrYGAExYAzAIYBXADY9e4AlhHSBQhA2ltWn1if1UweloEMgptDEgwAhwUHGIMBGICchowOBAbBxc3AHMxCX8yoSSoKC5SsF9FLAbBTRrILgAnflatFp6ahCyO7sSvCrGB6Xp4LgBjD3GJJT8A7XgEkDZiaDICJGRxQgxaMlowQRg4bishFe875sb8uydXLqEY1c-BfuZ4joAd0e3wErQqHT4aVa9y+TRqMjk0QahAOKOIGmI4FgkAIe0gmWIYAQyCQtFYbF+eQKbxKPyqOHpYPaH2hIJJMKwEjwI0qeEE4nAJIRs2B-XFnimXDFkv68L+0pBcq+CoASvTlQK1gwKhZtdBaFx1YKkFhBel2iBdZaYIb1RpWtVaAoGeAhIwcAahF6AtA8VwwBhGSazdJhYyaUU2M4WLcIPqElwAA6Lb6TZnx2r1SUZyWkKS1ADKVuiIIdIMCDmcAGcWDUcFxi87GQ9lC2ITUq7X64ryw9m98Ouq20ItCPVIy1tb9YbSiP4fPqgFkyXOauQPm3cvShyN5BRLviHgWQB7T50BoIxQILB6nGbQIAWQAwgBVAA61YA4gAFACSADyXBnsYF6TvwURYHIhChOS5L7lEIhcvemYQM+ACCP6fjwJ6fgAQuhhYAKLAUIzCXj4kHYsoJyGAgAhkhoFzIfAmYtgwWIADJEQAIlw+jgEgIAAGIQHqlhbBAPGODwACen7PrJcwxlwQA

[Lucky-Luka]

ok. I was just curious about the polarity of the cap and the utility of that diode there

[mikerj]

The capacitor does appear to be backward in the original schematic.  The left hand side cannot go above one Vbe, whereas the right hand side will be at 5v with the cap fully charged, so the positive side of the cap should be on the right.

Not entirely sure what the function of the diode is, I initially though it was to discharge the cap more quickly when the +5v supply is removed, but it has negligible impact since the 150k dominates.

[Lucky-Luka]

OK, asking other people about the watchdog and thinking about the overall project myself I've redesigned the circuit using an Arduino Nano board.
Tell me what you think about it.
Thanks

[pqass]

Your new placement of the diode in the WD part should make sure you don't reverse the polarity of the cap after it's discharged (ie. right-side never goes lower than V_be).  However, in the simulation, the diode drop was only about 430mV. Add to that the 23mV of the on-state transistor drop and it's not quite enough to keep the cap from being reversed (by about 130mV). Simulation isn't the "real world"^TM so confirm in practice that the diode you select has V_f > V_be or use two diodes for good measure.  I don't know if applying just 130mV of reverse voltage on an electrolytic is really bad in practice.  You shouldn't be keeping that GPIO line high for an extended amount of time anyway.

As for diode protection of the regulator, Google image search "lm317 diode protection".  You'll see another [reversed] diode between output and adjust pins.  And also a 10uF cap from adjust pin to ground.