if one circuit is in use and another calls ?

[plasticcat]

ok here is my question.... like most i have it in my head but hard to put in words so heres a shot...

i am making a circuit that lets 2 thermostats controll one air conditioner unit and i have a relay that allows one or the other thermostat to be sent to the main air conditioner at any given time.... so lets say thermostat 1 is on the normally closed contact on the relay and thermostat 2 is on normally open, now lets say if this condition existed..... thermostat 1 is in use at this given moment then thermostat 2 is turned on and being used. I cant have 2 thermostats calling at the same time, so i have a astable timer circuit in place to give 15 minutes to each thermostat untill each thermostat has reached temp. ok so far so good, but lets say thermostat 1 (normally closed) is calling first and then thermostat 2 (normally open) calls therefore energizing my timing circuit to alternate 15 minutes between them both, but in doing so, in the nature of the circuit, it will enegize my relay allowing thermostat 2 priority when thermostat 1 should have it.......

i hope i made sense

so to solve this i have came up with this solution...

in the event that both thermostats are calling and thermostat 1 calls first, i have a secondary monostable timer circuit set at 15 min to allow the full 15 minutes to thermostat 1 is allowed to use then when the 15 minutes is over, turn controll over to the other timer circut to go into normal 15 minute cycles

god i hope this made sense, im really bad at typing out whats in my head

if this sounds like it would work give me input if not tell me how i can explain better

 

[Boncuk]

Hi plasticcat,

you're overcomplicating your project.

If you have two temperature sensing circuits why not have them energize their loads independently?

Normally a dual OpAmp configured as comparator will do the job nicely.

Here is a basic schematic.

Boncuk

 

[BrownOut]

Because it's the same load for both thermostats, if I understand the OP. But why use two thermos? Are you trying to get an average from both? If your multivibrator is truly astable, then it shold not be triggered by either thermo, but run constantly, switching from one to the other at 15 minute intervals. No secondary multivibrator would be required.

Otherwise, if you want to run unit #1 constantly until unit #2 calls, but hold off switching for 15 minutes after #2 calls, then that's another story. You might want to use a dual timer for that purpose. What device are you present using to make your multivibrators?

 

[plasticcat]

what im trying to do is lets say a house has a upstairs and a down stairs, with one air conditioner, a thermostat for up stairs and downstairs, think of the thermostat you have in your house on the wall

each t stat has 3 wires i need to choose between ( heating, cooling and a fan ) i cant have both thermostats sending information at once so i have 3 relays to switch them, depending on if the relays coils are enegized, now i need a timer circuit that alternates them every 15 min if both are calling at once, so basically if normally closed thermostat calls first, then normally open thermostat calls second and starts a basic astable timing circuit the first cycle will be on therefore giving the normally open thermostat priority because the coils will be energize, its supposed to work by giving what ever thermostat called first 15 minutes then cycle,

here is a diagram of how the relays are set up and the coils are gonna be controlled by the timing circuit and the timing circuit is gonna be turned on when any 1 wire on each thermostat has power at the same time

basically the relays switch complete controll of the a/c unit depending on if the coils are energized or not

 

[vne147]

Other than the obvious answer of temperature, what are you trying to control? Are the two thermostats in seperate zones? Are they redundant to one another? Do you want both thermostats to come up to temperature before the heater turns off? Do you want both thermostats to drop below temp before the heater turns on or is just one thermostat below temp enough to turn the heater back on? The astable timer might be unnecessary depending on what it is exactly you're trying to accomplish. Can you make a table that shows the different states the system can be in and whether you want the heater on or off for each state? I think it will help to make your explaination much clearer. Something along the lines of this:

**broken link removed**

The above it just an example. You'll have to fill in the blanks according to your requirements.

EDIT: At what voltage and current do the thermostats operate. You could possibly use logic gates to accomplish what you want.

 

[plasticcat]

that table is exactly right, but you also have to add in air conditioning as well,

the thermostats run at 24v AC

 

[BrownOut]

Ok I get it. You just need two 15 minute timers, and allow each to disable the other when active. It's not hard to do with a simple 555 timer, or other suitable timer device. Don't use an astable configuration, just configure it as a monostable, or one-shot with an active period of 15 minutes. If you need help making the configuration, I could scrawl out a schematic later, when I have more time. Or else look up "monostable timer" or some such text on google to see how it's configures. Or else look at **broken link removed**

 

[BrownOut]

Other than the obvious answer of temperature, what are you trying to control? Are the two thermostats in seperate zones? Are they redundant to one another? Do you want both thermostats to come up to temperature before the heater turns off? Do you want both thermostats to drop below temp before the heater turns on or is just one thermostat below temp enough to turn the heater back on? The astable timer might be unnecessary depending on what it is exactly you're trying to accomplish. Can you make a table that shows the different states the system can be in and whether you want the heater on or off for each state? I think it will help to make your explaination much clearer. Something along the lines of this:

**broken link removed**

The above it just an example. You'll have to fill in the blanks according to your requirements.

EDIT: At what voltage and current do the thermostats operate. You could possibly use logic gates to accomplish what you want.

I think he wants to switch because one might be calling for cooling and the other might be calling for heating.

really though, it that's the case you want something more complex, as you don't want to heat-cool-heat-cool-etc.

 

[plasticcat]

i can build the timers thats easy, but my main problem is when the timer circuit is first started the first cycle is on, but what if the first t stat that is called needs to be in the off cycle, thats where my second timer comes into play

 

[BrownOut]

If you use a monostable, it won't start until it's corresponding t-stat calls. Both timers will be just sitting and waiting until someone calls. Then it will start and run for 15 minutes, then go back to sleep until someone calls again.

However, at the end of the 15 minutes, if the t-stat is still calling, you'll get a little "glitch" if that t-stat is connected to the NO pin of your realy. You don't really want that, so I guess you'll want to leave the relaly on until the t-stat stops calling.

 

[plasticcat]

no see if only one is calling it will run for ever there is no need to time it, the timing is only when both of them are calling because then it needs to "share" the one a/c unit if only one or the other calls it can run for ever no need to share, basically what im trying to accomplish is if 2 of them call, i need the one that called first, to get its 15 minutes first reguardless if its on normally open or closed part of the relay, and i need astable so it retriggers the cycle of on and off forever untill one of the thermostats stops calling

 

[BrownOut]

The function of each t-stat leg should be:

Output = calling OR Timer. Where timer is a monostable. That gets you where you want to be.

 

[Boncuk]

I think he wants to switch because one might be calling for cooling and the other might be calling for heating.

I'm facing that problem daily. At 30deg/C my wife calls for heating while I call for cooling.