An update from my previous posts before the other thread was locked. Per comments from several other members, and the realization that the two relays being used (P&B RTE24012F and Omron G6A-234P) did not have the same pinout, i modified the design to still allow either relay type to be used by adding additional jumper pads. I did not post the updated schematic file, so don't use that one (I will post the correct one next week). The board is still approximately 3" x 2.2", and features the following additions:
1) Provision for a manual mute switch bypassing the delay
2) Addition of fault/mute and on LEDs
3) Use of a 555 timer instead of the resistor/capacitor time constant for driving the relay and setting the delay. A 1M Rt and a 4.7uf Ct gives a dalay of about 5.1 seconds, etc. I believe the delay may be calculated by 1.1 * Rt * Ct. This is independent of the relay turn on characteristics.
4) Will accomodate either the newer Omron relay package (smaller) or the original Tyco form factor relays, making it easier to source
5) Does not employ the pre-packaged Darlingtons, but rather 2N3904s and (1) 2N 3906 to make parts sourcing easier (E-B-C TO-92 package). Packaged Darlingtons could certainly be used if you have them laying around.
6) Configurable relay: Per some concerns (not mine) about running the audio signal thru a relay, there are jumpers which allow the relay to be configured to short the output, with the audio not going thru the relay (unless shorted). Provisions for paralleled back-back BAT43 Schottkys are there if you don't want a dead short. As Amb noted, this will limit the offset seen on the output to 0.3V (300mV), so low impedence cans will not be completely protected in this configuration. Note that on this board layout, the Schottkys will need to be stacked as I ran out of room to handle them individually... I don't think many will use this configuration, but they are there if you want to do it that way...
I have a prototype partially built and will try to post a pic of it (and compared to Guzzler's PCB to show size difference).
Schematic:
Board:
Eagle files here:
board
Muting delay and offset protection circuit (Amb, modified)
I don't have any electroless tinning stuff, so I just coated the board with flux and hand tinned it with the widest tip I had... looks crappy but it works.
).
