# Driving a SPDT switch

Discussion in 'The Projects Forum' started by Babae, Feb 5, 2014.

1. ### Babae Thread Starter New Member

Feb 4, 2014
4
0
I am trying to think of a simple circuit that drives the control voltages of a SPDT switch. The switch has two control voltages V1 and V2.

State 1: V1 high and V2 low -> RF1 on and RF2 off
State 2: V1 low and V2 high -> RF1 off and RF2 on
State 3: V1 high and V2 high -> RF1 on and RF2 on
State 4: V1 low and V2 low -> RF1 off and RF2 off

The logic voltage level is around 4V. I want to switch between State 1 and State 2 every 50usec or so. Can anyone give me an idea on how to implement this?

I had two ideas. Using a flip-flop or a LM555 timer circuit. Though, I haven't figured out the specifics yet.

2. ### AfdhalAtiffTan Active Member

Nov 20, 2010
117
11
What kind SPDT switch did you have? A relay?
If you want to change between states, using a diode OR-GATE for each state might be possible.

3. ### ErnieM AAC Fanatic!

Apr 24, 2011
7,906
1,789
What happened to States 3 and 4????

Have you noticed the strange synchronicity between 1: V1 and RF1? Same between V2 and RF2

50uS is just 20KHz. A 555 running at 40KHz driving a flip flop would make a perfect 50%-50% square wave as long as States 3 & 4 are ghosts.

4. ### burger2227 Member

Feb 3, 2014
194
25
You'll need something to create a timed pulse so you'll at least need one 555 to do anything. The flip flop would need a trigger pulse.

5. ### #12 Expert

Nov 30, 2010
17,829
9,153
This sounds like: generate 10KHz. Use that to drive something that outputs "Q" and "not Q". A 555 timer can do the frequency. A flip/flop chip can do the Q and not Q.

For any "equal to or greater than 4.0 volts" at V1, RF1 becomes "High". At the next clock event or some other time later? How much higher than 4.0 volts?
For any "equal to or greater than 4 volts" at V2, RF2 is "High".
For any V1 less than 4.0 volts, RF1 = "low". How much less than 4.0 volts? How much later?
For any V2 less than 4.0 volts, RF2 = "low".

How high is, "high"?
How low is, "low"?
How much current is needed?
What voltage power supply is available?