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From: baron on 15 Oct 2009 08:46
Hi Guys,

I'm doing a hobby project to build a timer that can be used to turn on a
relay at a pre-determined time and then turn it off after a delay.
I also want to use a temperature sensor to override the relay but only
during the time period that is programmed.

The idea is to turn on a water heater for a pre-determined period but to
use a thermostat to control the temperature during this period.

I have suggested using a relay because the heater is rated at 3.5Kw
240/250 volts, 50hz AC. Low voltage relays are available that can
handle this load with ease. The heater has a safety cutout built in
that should prevent boiling dry.

I am aware that I can buy programmable motor driven timers very cheaply,
however they are limited in contact loading and I would still need a
relay for thermostat control.

I thought this might make a nice PIC project. (Ducking) :-)

--
Best Regards:
Baron.
From: PeterD on 15 Oct 2009 09:25
On Thu, 15 Oct 2009 13:46:48 +0100, baron
<baron.nospam(a)linuxmaniac.nospam.net> wrote:

>Hi Guys,
>
>I'm doing a hobby project to build a timer that can be used to turn on a
>relay at a pre-determined time and then turn it off after a delay.
>I also want to use a temperature sensor to override the relay but only
>during the time period that is programmed.
>
>The idea is to turn on a water heater for a pre-determined period but to
>use a thermostat to control the temperature during this period.
>
>I have suggested using a relay because the heater is rated at 3.5Kw
>240/250 volts, 50hz AC. Low voltage relays are available that can
>handle this load with ease. The heater has a safety cutout built in
>that should prevent boiling dry.
>
>I am aware that I can buy programmable motor driven timers very cheaply,
>however they are limited in contact loading and I would still need a
>relay for thermostat control.
>
>I thought this might make a nice PIC project. (Ducking) :-)

If you are doing this for a fun project, yes, that will work.

If you want to create a good, dependable system, look at timers made
for high current loads, such as swimming pool filter systems. Or use
that relay (which you will need for your fun project anyway).

KISS--Keep It Simple, Small.
From: don on 15 Oct 2009 10:16
baron wrote:
> Hi Guys,
>
> I'm doing a hobby project to build a timer that can be used to turn on a
> relay at a pre-determined time and then turn it off after a delay.
> I also want to use a temperature sensor to override the relay but only
> during the time period that is programmed.
>
> The idea is to turn on a water heater for a pre-determined period but to
> use a thermostat to control the temperature during this period.
>
> I have suggested using a relay because the heater is rated at 3.5Kw
> 240/250 volts, 50hz AC. Low voltage relays are available that can
> handle this load with ease. The heater has a safety cutout built in
> that should prevent boiling dry.
>
> I am aware that I can buy programmable motor driven timers very cheaply,
> however they are limited in contact loading and I would still need a
> relay for thermostat control.
>
> I thought this might make a nice PIC project. (Ducking) :-)
>

I have built and programmed a battery backed clock chip and a PIC16F628,
attached to a RS-232 serial port.

I am sure this can be modified to add a relay driver.

Let me get the docs together sometime this weekend.

don
From: baron on 15 Oct 2009 10:52
don <don> Inscribed thus:

> baron wrote:
>> Hi Guys,
>>
>> I'm doing a hobby project to build a timer that can be used to turn
>> on a relay at a pre-determined time and then turn it off after a
>> delay. I also want to use a temperature sensor to override the relay
>> but only during the time period that is programmed.
>>
>> The idea is to turn on a water heater for a pre-determined period but
>> to use a thermostat to control the temperature during this period.
>>
>> I have suggested using a relay because the heater is rated at 3.5Kw
>> 240/250 volts, 50hz AC. Low voltage relays are available that can
>> handle this load with ease. The heater has a safety cutout built in
>> that should prevent boiling dry.
>>
>> I am aware that I can buy programmable motor driven timers very
>> cheaply, however they are limited in contact loading and I would
>> still need a relay for thermostat control.
>>
>> I thought this might make a nice PIC project. (Ducking) :-)
>>
>
> I have built and programmed a battery backed clock chip and a
> PIC16F628, attached to a RS-232 serial port.
>
> I am sure this can be modified to add a relay driver.
>
> Let me get the docs together sometime this weekend.
>
> don

Thanks. I'll take any help I can get.

--
Best Regards:
Baron.
From: John Fields on 15 Oct 2009 10:58
On Thu, 15 Oct 2009 13:46:48 +0100, baron
<baron.nospam(a)linuxmaniac.nospam.net> wrote:

>Hi Guys,
>
>I'm doing a hobby project to build a timer that can be used to turn on a
>relay at a pre-determined time and then turn it off after a delay.
>I also want to use a temperature sensor to override the relay but only
>during the time period that is programmed.
>
>The idea is to turn on a water heater for a pre-determined period but to
>use a thermostat to control the temperature during this period.
>
>I have suggested using a relay because the heater is rated at 3.5Kw
>240/250 volts, 50hz AC. Low voltage relays are available that can
>handle this load with ease. The heater has a safety cutout built in
>that should prevent boiling dry.
>
>I am aware that I can buy programmable motor driven timers very cheaply,
>however they are limited in contact loading and I would still need a
>relay for thermostat control.
>
>I thought this might make a nice PIC project. (Ducking) :-)

---
View in Courier:


+----------+
| C O------|-----+
| | NC | |
| > |<--O--|--+ |
+----------+ | |
TSTAT | |
| |
| |
+-------------|--|-----------------+
| TIMER | | |
| +------+ | | |
AC>--+-| |----+ +---+ |
| | | NO |
| | [COIL]- - -| <--O
| | | |
AC>--+-| |-----------+ OC
| +------+ |
| [HEATER]
| |
+-----------------------------+


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