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udailey 4th October 2011 02:51 PM

LDR Based Input Selection
 
A few years ago Jon VerHalen of Lowther America bought some LDRs from me and asked if I had ever thought to use them for input selection. He sent me a simple schematic with a series LDR and a relay after it. The idea is to let the LDR go dark, 25Megohms, and shunt anything that gets through to ground. I added a series LDR after the relay because if I am sending signal from another source I dont want it to get shunted to ground by the unselected source's relay. That and a good power supply is about all you need. So it goes :
Series LDR/shunt to ground/Series LDR
25Megohm/5ohm/25Megohm for non-selection of a source
40Ohm/infinite Ohm/40Ohm for selection of a source
Yes there is series resistance but it just ends up being part of the series resistance of your attenuator anyway and 80R is nothing to a 10k pot. If you turn your attenuator up all the way you are only missing the last 1% of volume but you have an incredibly great sounding input selector.

udailey 4th October 2011 02:52 PM

2 Attachment(s)
Some pics in various stages of completion

udailey 4th October 2011 02:59 PM

If you want to view the entire album, here is the link to my Picasa web album.
https://picasaweb.google.com/udailey...eat=directlink

mickeymoose 4th October 2011 03:14 PM

Looks good, could you post the schematic? E

udailey 4th October 2011 03:18 PM

The schematic has always been in my head and I never drew it. I can do a simple one real quick for you to get the idea. Will sketch and take a pic.

udailey 4th October 2011 03:32 PM

1 Attachment(s)
Here is the schematic. Simple as it may be.
This is the general idea. Regulated voltage is then put through a current source which supplies power to the LDRs. Regulated voltage is applied to a trimmer that turns on the transistors. By twiddling the trimmer you can balance the resistance of the LDRs so they are the same. By increasing the voltage you can lower the resistance of both at the same time.
ON means both LDRs turn on full force at 40R each for a series total of 80R and the relay is open.
OFF means that both LDRs have 25Megohm or more resistance and the relay has less than 5R. Its an optocoupler relay to keep with the theme.

udailey 4th October 2011 03:35 PM

Of course there's lots of caps and a few resistors to limit current to the optocoupler relays as well as a 249k between V+ and the trimmer.
Really thats about it. Just copy this circuit twice for each input and then give V+ to that trimmer to turn on the circuit and select that source. Take V+ away and the input is not selected.

udailey 4th October 2011 03:36 PM

You could also use it for 2 balanced sources or 1 balanced source and 2 single ended.

udailey 4th October 2011 03:38 PM

OH, we have not traditionally had a MUTE function with LDR attenuators. If you couple this with an LDR attenuator all you have to do is use a rotary switch to send V+ to the different source selection circuits. If you have one extra position on your rotary switch then just dont connect that to anything and when you switch to that position you get MUTE.

CharlieLaub 4th October 2011 03:57 PM

A pair of LDRs makes for a great switch. I think the traditional "switch" application for LDRs is just a simple shunt where both elements are LDRs. Either one is turned on or the other. This gives two states: very high input impedance (like 25M ohms) and a very low impedance between that and ground (e.g. the OFF state) or very low input impedance and a very high impedance between that and ground (e.g. the ON state).

Using LDRs as 100% on/off devices results in very low distortion IIRC according to the application note. Much less than when used as an attenuator.

I can't follow why you used this particular circuit (in post #6) for switching...

-Charlie


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