Auto power on/off switch for RF signal

[Julie]

I have hooked up a secondary (12 volt, low-power) monitor to an occasional RF
output. Currently, the monitor is always on, however, I'd like to be able to
automatically switch the monitor off after a period of RF inactivity.

I'm looking for a circuit design that will monitor the RF signal, and if
present, switch (12v) power on; if not present, switch power off.

Any simple circuit that I can construct, or is there anything out there
pre-made?

Thanks for any info.

 

[John Fields]

On Wed, 12 May 2004 12:03:45 -0700, Julie <julie@nospam.com> wrote:

I have hooked up a secondary (12 volt, low-power) monitor to an occasional RF
output. Currently, the monitor is always on, however, I'd like to be able to
automatically switch the monitor off after a period of RF inactivity.

I'm looking for a circuit design that will monitor the RF signal, and if
present, switch (12v) power on; if not present, switch power off.

Any simple circuit that I can construct, or is there anything out there
pre-made?

---
If you've got enough RF, probably the easiest thing to do would be to
build a simple AM diode detector and use its output to trigger a
comparator wired to turn on a relay(?) and hold the relay on until a
little while after the RF goes away.

1. What frequency are you talking about?

2. What RF amplitude do you have available?

3. What's the output impedance of the RF source?

4.Do you want a schematic?

--
John Fields

 

[Julie]

John Fields wrote:

On Wed, 12 May 2004 12:03:45 -0700, Julie <julie@nospam.com> wrote:

I have hooked up a secondary (12 volt, low-power) monitor to an occasional RF
output. Currently, the monitor is always on, however, I'd like to be able to
automatically switch the monitor off after a period of RF inactivity.

I'm looking for a circuit design that will monitor the RF signal, and if
present, switch (12v) power on; if not present, switch power off.

Any simple circuit that I can construct, or is there anything out there
pre-made?

---
If you've got enough RF, probably the easiest thing to do would be to
build a simple AM diode detector and use its output to trigger a
comparator wired to turn on a relay(?) and hold the relay on until a
little while after the RF goes away.

1. What frequency are you talking about?

RF TV channel 3/4.

2. What RF amplitude do you have available?

Don't know, I haven't scoped it and it is from a consumer appliance, so it
isn't in the specs. I'd suspect 1v p-p (?).

3. What's the output impedance of the RF source?

No idea. It is the output from a Sony TiVo SVR-2000 DVR.

4.Do you want a schematic?

Yes, please!

--
John Fields

 

[Julie]

Julie wrote:

I have hooked up a secondary (12 volt, low-power) monitor to an occasional RF
output. Currently, the monitor is always on, however, I'd like to be able to
automatically switch the monitor off after a period of RF inactivity.

I'm looking for a circuit design that will monitor the RF signal, and if
present, switch (12v) power on; if not present, switch power off.

Any simple circuit that I can construct, or is there anything out there
pre-made?

Thanks for any info.

Anyone have any ideas on this one? Still looking for some suggestions or
potential solutions.

Thanks

 

[Rich Grise]

"Julie" <julie@nospam.com> wrote in message
news:40A5760C.1024E85E@nospam.com...

Julie wrote:

I have hooked up a secondary (12 volt, low-power) monitor to an
occasional RF
output. Currently, the monitor is always on, however, I'd like to be
able to
automatically switch the monitor off after a period of RF inactivity.

I'm looking for a circuit design that will monitor the RF signal, and if
present, switch (12v) power on; if not present, switch power off.

Any simple circuit that I can construct, or is there anything out there
pre-made?

Thanks for any info.

Anyone have any ideas on this one? Still looking for some suggestions or
potential solutions.

Thanks

When the power's off, how does it know that there's a signal?

Thanks,
Rich

 

[John Fields]

On Fri, 14 May 2004 18:44:44 -0700, Julie <julie@nospam.com> wrote:

Julie wrote:

I have hooked up a secondary (12 volt, low-power) monitor to an occasional RF
output. Currently, the monitor is always on, however, I'd like to be able to
automatically switch the monitor off after a period of RF inactivity.

I'm looking for a circuit design that will monitor the RF signal, and if
present, switch (12v) power on; if not present, switch power off.

Any simple circuit that I can construct, or is there anything out there
pre-made?

Thanks for any info.

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] [10ľF] [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 10ľF 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.

--
John Fields

 

[Earl]

Julie,

The RF channel 3/4 output isn't likely enough to run a diode detector. It's
meant to attach to the input of a TV.

Maybe you could take a tuner from a TV set. There are plenty of those in
the junkyard with working tuners. You could just look at the AGC line, or
video output, or something. An RC circuit could provide the turn-off delay.
--
Earl

"Julie" <julie@nospam.com> wrote in message
news:40A27511.125DB74C@nospam.com...

I have hooked up a secondary (12 volt, low-power) monitor to an occasional
RF
output. Currently, the monitor is always on, however, I'd like to be able
to
automatically switch the monitor off after a period of RF inactivity.

I'm looking for a circuit design that will monitor the RF signal, and if
present, switch (12v) power on; if not present, switch power off.

Any simple circuit that I can construct, or is there anything out there
pre-made?

Thanks for any info.