How do I switch resistance?

[SgtB]

I have a couple of digital pots on the way, but I was wondering what kind of transistor or gate would I use to achieve the same thing? I'm building a translator for the controls in my car for an aftermarket pioneer radio. It uses the Sony wired remote format which is just a bunch of resistors in series.

The radio sends out a 5v signal to measure the resistance. I'm having a hard time wrapping my head around replacing those switches with logic gates. I don't know if I should be calculating the internal resistance of the gate , if that's even possible, or if there is an optimal component for this. My bench work has been hit or miss, and I think I need some advice on switching analogue.

One other thing. Can I use an opti-isolator and pwm to achieve the same thing?

 

[BCJKiwi]

Not quite sure what the question is.

If using the digital pot to replace the remote, it needs to be configured to produce the same resistance as seen on the jack for each switch position.

If replacing the manual PBs with transistor or gate, one would be required in place of each existing PB.

Resistance of the switch/transistor/gate should not be a factor unless significant in relation to the resistance.

See the discussion on 'ADC or One input Keypad decoder' in the forum - do a search

Just a caution on the Shift key - there is no resistor in the switch circuit.

 

[SgtB]

Simple operator error. I had a few wires crossed and was stumped. I just dropped in some 2222a's and it's working good now. Thanks.

 

[moxhamj]

If one of the lines is ground then transistors are certainly an option. Alternatively have a look at the 4016 quad analog switch package. 30c from Futurlec http://www.futurlec.com/4000Series/CD4016.shtml

If you want to control 8 switches, and only one switch is on at any one time (which would be true for a resistor chain like this), then maybe look at the 4051. This can switch 8 lines (join the common to ground) but instead of using 8 control lines, you only need 3.

If you want to switch 16 lines, you could do this with two 4051 chips and 3 control lines, plus two more lines, one to each of the inhibit inputs of each 4051. So you could switch 16 switches with only 5 control lines.

 

[kewakl]

Dr!
i am familiar with the 4066. What are the more significant differences to the 4016?
I am downloading the datasheets but am on dial-up, so it may be a while.

 

[moxhamj]

Different "on" resistance I think. See the data sheet for the exact values. The 4066 is lower - 80 ohms. The 4016 is 400 ohms. The 4051 is 80 ohms. Hopefully that is ok.

Then of course, there might be a cunning circuit that only uses one transistor or one 4066 gate. If you turn an 80 ohm resistor (or any value for that matter) on and off fast enough, if you use 80R at 50:50 duty cycle, it will turn into a 160R. Thus you could emulate all the switches by changing the pulsewidth. Maybe worth a try?

 

[SgtB]

Thanks. I was looking at the 4016 last night! I may order a few for later, but I was trying to work with what I had. I'm using some generic npn transistors and resistors. It's a mess, but It's working. I ended up only needing 6 circuits anyway, as my radio doesn't support all the buttons. I was using a shift register and a 08M, but I needed one more input and jumped to a 18x. At this point I'm scared to touch it.