Anyone have any ideas on this one? Still looking for some suggestions or
potential solutions.
---
From your previous reply you indicated the device which is generating
the signal is a SONY SVR-2000. Since you say your monitor's plugged
into the RF output, that leaves the video output vacant, which you
could use to determine when there's signal coming out of the RF port
without actually having to tap into the RF, which I doubt is 1VPP.
Try this: (View it in a fixed pitch font like Courier)
+V>------------------------------------+-------+----+---------+
| | | |
| | | +---+---+
| | | |K |
| | | [1N4001][RELAY]
[RS] | | | |
| | [RL] +---+---+
| | | |
VIDEO>--[100pF]-+-[1N4148>]-+-----+----|------|+\ | C
|K |+ | | | >--+--[1K]--B 2N4401
[1N4148] [10µF] [1M] [100K]<--|-/U1 E
| | | | | |
GND>------------+-----------+-----+----+-------+--------------+
+V can be whatever you have lying around, up to about 36V, the max
allowable for U1, (1/2 of an LM393; short the inputs and the output of
the other half to ground), and size Rl to allow no more than about 4
mA to flow into the comparator's output when it's low. For example,
with a +V of 5V, RL = E/I = 5V/4mA = 1.25k ohms. 1.2k would be OK, and
it would dissipate P = I*E = 4mA*5V = 20mW, so a standard 1.2k +/- 5%,
1/4 watt resistor would be OK.
With 1VPP of video in (6MHz sinewave) the + input of the comparator
sees about 400mV of DC, and with the 10µF cap and the 1 megohm
resistor shown, it decays to about 150mV in 10 seconds.
You can use the 100k pot to adjust the timeout to much longer than 10
seconds if you want to by cranking it closer and closer to ground.
Use RS to adjust the sensitivity of the pot so you don't waste a lot
of rotation just to get to where the pot starts to work. For example,
1 volt across the pot would probably work pretty well, so you can
figure out RS by considering that the pot and RS look like a voltage
divider:
E1
|
[R1]
|
+---E2
|
[R2]
|
0V
Now, if E1 is +V (and it's 5V), R1 is RS, and E2 is the voltage we
want with the pot (R2) cranked all the way up, we can write:
R2 (E1-E2 100E4 (5-1)
R1 = --------- = ------------ = 400E4 = 400000 ohms
E2 1
430k is a 5% value and would put E2 just a little under 1V (943mV), so
that would be OK.