Hello all, I found a circuit that uses the CD4066 quad analog switch that=20 works well. The on resistance at 12 volts is about 32 ohms. The off=20 resistance is more than 10 megaohms. I see two draw backs with this=20 circuit. The first is the 32 ohm on resistance. This means that the=20 input resistance to the down stream circuit must be pretty high.=20 Probably over 10K would be best. The other is that it requires 8=20 switches, 2 chips to switch two circuits. Each additional circuit=20 requires an additional 4 switches or one chip. Two switches are connected in series. One switch is used to ground=20 the output of the first switch when the circuit is in the off state. The=20 fourth switch is used to control the gating of the first two switches. I=20 could add another switch in parallel with the output switch this will=20 drop the output resistance to under 16 ohms. In fact if I were to put=20 two switches in parallel the output resistance would drop to under 8=20 ohms. This would mean that it would take three chips for every two poles. The on resistance of one switch at 12 volts is around 16 ohms and=20 the off resistance is about 30K ohms. So this design increases the the=20 off resistance by a factor of over 350 times. It only doubles the on=20 resistance. So this is a good circuit. The complication is not a big=20 deal as these chips are only about 50 cents each. So even after adding a=20 socket , the three pullup resistors, and a decoupling capacitor for each=20 chip the cost is still only about $1 per pole. With three switches in=20 parallel the cost would be about $1.50 per pole. This a lot cheaper than=20 rotary switches that can perform the function. This is also a lot more=20 reliable since the only current flowing through the rotary switch=20 contacts will be only a few milliamps at 12 volts. Thanks everybody for all of your help. If anyone has any other=20 suggests please feel free to weigh in. Thanks, rich! On 5/23/2014 3:23 AM, alan.b.pearce@stfc.ac.uk wrote: >> Joe, Steve, >> Thank-you for the suggestions. This is what I am looking for. Righ= t >> now I am experimenting with a CD4066. I don't have any 4051s right now >> but the 4066s are helping me to design a control system. >> rich! > Another one to look at is the venerable DG403 series of chips. Has logic = level control, but the switch can deal with +/-12V (when using appropriate = supply rails). Main disadvantage is it is a 3 rail supply (5V logic and +/-= 12V analogue). IIRC the DG403 has 2 switches in a package, I don't know for= sure if there are others in the family have more, but I have a feeling the= re is a quad version. --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .