PIC input selector ( preamp ? )

[breguetphile]

I have recently programmed a pic 16f84 to decode a sony remote and I was wondering what the best way to switch between input sources is. One of my profs recommended using the analog cd4066, but I was wondering if relays would do the job better. I currently have the outputs I need all outputting as "one-hots" so I could easily fire a relay to switch. Is this the best way or is there a better way? What is the best way to control the volume remotely? (is it the relay based APOX I read about?)

Thanks

[Andypairo]

IMHO a relay will do the job better, since it contains no active elements.
Does your code provide latching momentary outputs ?
I'd like to have a look at the schematic you are using and, if possible, to have access at the firmware since I have some of these chips lying around doing nothing....

Cheers

Andrea

[breguetphile]

You'll have to excuse anything I did poorly as this was my first pic programming. I will take any suggestions for improvement.

I have the source for the decoding here:
www.brentcastle.com/irdec.c

schematic is here:
www.brentcastle.com/irdec.png

As you can see from the code I give very loose counts for high and low (these are pulse-width defined logic pulses). If you are curious the start bit should be around 165 (which i just said i was looking for >140) , the '0' bit is about 47 timer ticks, and the '1' bit is about 75 timer ticks. As stated before this is to output a one-hot. In other words, pressing a 1 will output (and latch) rb7, pressing 8 will output and latch rb0. This code was compiled using picclite and is for a 16f84a.

[jackinnj]

but there are some very good switches from Analog Devices, Maxim. I would completely avoid the generic CMOS switches.

if you are going to use relays, consider using a relay driver like the ULN2803a -- each gate will drive 500ma.

[breguetphile]

Allright, now that I know what driver is recommended could anyone give me advice on what relays are good? I'm not really sure how much of a difference it makes in audio. I must admit I do like the idea of the input selection being audible (with a nice click!!). I suppose I'd do the same thing with the volume control. Are there any reasonably priced relays that I could acquire in bulk to handle both of these jobs?

Thanks!!

[Jan Dupont]

Hi,
Last week I started the design of a preamp also using a PIC (as input selector) and analog switches for the inputs.

The idea is taken from the exelent QUAD 34 and 44 preamps which I got a couple of....

QUAD uses the 4066 analog switch, and even that this circuit has been on the market for a long time, the s/n ratio and channel separation is quite good.

With the QUAD design in mind (and 20 years experience with bad and noisy relays) I have layed my eyes on the SSM2402 and SSM2404 analog switches from Analog Device. With a s/n ratio of up to 120 dB and a COMPLETE clickless operation I think this is the most obvisous choice, when designing a new High End preamp.

Don´t misunderstand. Relayes are good!!!! However having a preamp with NO MECHANICAL CONTACTS what so ever in the signal path is just great.

[mlloyd1]

There is probably some useful stuff in this post from the past

The Hafler 915 preamp (full manual with schematic on Hafler web site, just do a search) and the Parasound PHP-850 preamps (schematic available on request from Parasound) use CMOS analog switches as input source selectors.

As preamps go, you can do much worse than these.

I use Maxim switches cause I have a BUNCH

mlloyd1

Originally posted by jackinnj
but there are some very good switches from Analog Devices, Maxim. I would completely avoid the generic CMOS switches.

if you are going to use relays, consider using a relay driver like the ULN2803a -- each gate will drive 500ma.

[Bjerre]

One issue at least with mechanical relays you should consider is to implement a make-before-break when changing input channel. By doing this a changeover does not include the possible annoying state 'all-inputs-off'.