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# Help with AND gate Switching

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#### TheFox

##### New Member
Help!! It's quite sometime since I had anything to do with those dangerous little electrons et al. However, I now find that all that stuff I learnt may come in handy - if only I could remember it!

My problem is this. I want to produce 5 different switches to operate 5 outputs. I only want the output high when both inputs are high (i.e AND gate?). Each switch has 1 independent input (from 5 different sources)and 1 common input to all 5 switches. The initial input voltage is 240v 60Hz and the final output voltage also needs to be 240v 60Hz. Does all that make sense? If anyone can help me I would be most grateful!

Relay logic ???

If I guess correctly...
You have a common switch and 5 channel switches.
You want each of the 5 outputs (all identical in operation) to operate when (and only when) the common switch AND the associated channel switch is made.

The common switch connects to Live, its switched out splits to all 5 channels, each channel has a switch then a load to Neutral.

Straight switches seem to achieve this? Relays (240v coils) could be used if the channel switches can't 'float' between live and neutral.

Seems too easy... I've missed something?

Thanx for the reply mechie. They say a picture paints a 1,000 words. Oh to be able to draw it! I'll try and describe it a little more to enable you to draw it and confirm if we're both on the same lines!

The inputs are as follows:

A - Input to all 5 switches
B - Input to switch no.1
C - Input to switch no.2
D - Input to switch no.3
E - Input to switch no.4
F - Input to switch no.5

Inputs A to F can all be different (i.e on or off) and operate independently of each other. All inputs would be standard mains 240v etc.

So
A+B=output switch 1
A+C=output switch 2
A+D=output switch 3
A+E=ouptut switch 4
A+F=output switch 5

Output from each switch is to operate a valve which is either open or closed (normally closed) Only want valve open if both switch inputs are high and therefore switch output is high.
e.g valve associated with switch 1 should only open if both A and B are high. Valves also operate at 240v.

My reasoon for thinking of using TTL is to avoid all the 'clicking' of relays and also for reliability over a period of many years.

Central Heating Zone control?

Hi Fox,
I am starting to see a five zone central heating system here...
A master time clock killing the whole system during the night etc, each of the 5 zones having its own room thermostat controlling a zone valve (shutting that zone down when the room is warm enough but allowing the rest of the house heating to continue).
Even if this isn't your application it may help to visualise things?

My last post (titled Relay Logic) would cover this scenario perfectly. The live supply going to the master clock switch, through the switch (switched live) to all five thermostats. Each thermostat switching its own zone valve (all zone valve neutral leads then return to mains neutral).

If you really need relays then small ones are available that are reasonably quiet and are rated for 100,000 operations (that's 2.5 months at one operation a minute or 11 years at once an hour, all day, every day!).

If you really want TTL logic then the circuit is easy but the input 240v AC to 5v DC isolators on every input and 5v to 240v isolators on every output will complicate the building a lot as you are ruling out relays (the easiest form of isolator). :idea: Maybe small transformers & rectifiers could be used on the inputs?

PM me (button below) if I can email diagrams to you?

Thanx mechie have done so!

Hi Fox,

Seems like SSRs (solid-state relays) would be the output device of choice for what you want to do, especially if you are want to be able to control the loads from a low-level dc circuit. You don't give a description of the loads you want to control.

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