Programmable Timer Controller Circuit for Homemade Pellet Burner

The following post explains a programmable sequential timer with controller circuit which may be used for automatically controlling a homemade pellet burner/boiler system. The circuit was requested by Mr. Vasilis.


The Request


Hi there Swagatam!

I have used your circuit on my homemade pellet burner that uses a 12V Ac motor for the feeder screw running 8 sec on, 30 sec off , and it works great, my idea for my project was similar to your circuit,I also have a monostable circuit istalled using the famous 555 ic for the igniter which is a 12V glow plug that stays on for 2-3 min which is enough to ignite the pellets, here is the fun part now, I run in to some problems during ignition, my idea for this project was for me to use a plc controller for all the action, money is short and i find building the circuit more interesting.My question is if i can produce a sequence of actions for the ignition process, when power on the circuit 8-10sec on time 30 0ff time for the feeder, at this point glow plug to stay on around 2-3 min, and the fan blower which is a 220Vac to pulsate for the shortest 2-3 sec on 10- 15 off to it's lowest speed if possible, when fire is detected using a LDR to activate the second circuit ( 8 sec On 30 sec off)and the fan blower to its regular speed via a relay..
I hope i explained everything correct!
Greetings from greece
Vasilis K




Hi Vasilis,

Thanks!

Can you provide your requirement in a step-wise manner because I am finding it difficult to understand the sequence in the above explanation.

If you can provide the circuit operating sequence serially with the relevant timing functions, then I can try working on it.





Hi Swagatam.

There is a room thermostat that I turn on, which gives power to 
the circuit.

1) Timer for the pellets- 15 seconds then off
2) Timer for the glow plug - 2-3 minutes then off
3) Timer for the fan - pulsing 2-3 seconds on then 30 seconds off 
(this needs to cycle until the photo cell is activated,).
4) Photo cell - when it sees fire, it is to stop the entire cycle.
Then the next cycle begins:
5) Timer for the pellets - 8 seconds on, 30 seconds off until the water
temp reaches 75 degrees C (the boiler will cut the power at this temp).
So every time power in on it should do the steps above, I hope it's clear!
Let me know if you need more info..

Thanks for your Help again!

Vasilis K



The Design

The presented design of a programmable timer controller circuit for homemade pellet burner system looks  quite complex, but actually it's not, the design is simply a chain of  a few monostable, and astable multivibrator stages configured sequentially.

The entire circuit can be understood with the following points:

The circuit is initialized by applying power via a thermostat, as soon as its powered, current passes through C1 and triggers the stage#1 monosatble.

T1 activates the connected relay and the relevant load, and also ensures that the reset pin of stage#2 and supply pin#8 of stage#3 gets grounded so that they stay deactivated.

After the set time elapses, T1 releases itself and the connected load, triggering stage#2 into action.

Now T2 activates switching ON the relay and the relevant load, T2 also makes sure that the reset pin of stage#3 is held grounded so that it doesn't activate.

Once the set time of stage 2 gets over, the connected load is switched OFF and stage 3 which is an astable now gets activated.

Stage three toggles the connected load ON/OFF at some specified rate as per the duty cycle set by appropriately selecting the value R16.

The above astable remains ON until the pellets ignite, and the light from its glow activates the comparator associated with stage 4.

With the detection of fire the opamp output goes low grounding the reset pin of stage 3 astable, thus deactivating its function.

The opamp triggering in the process also activates another astable at stage 5 which just like stage 3 toggles a connected load at some specified rate determined by the evaluation of R24.

The last stage is responsible for heating up the boiler water whose temperature is monitored by the thermostat. Once the water temperature  reaches the set degree, the thermostat switches OFF power to the entire circuit so that everything is reset back to the original state, ready for commencing a fresh cycle.

Feedback from Mr. Vasilis K:

Hello swagatam vasilis k here.

 I have started putting the circuit together on a breadboard then i realized my mistake at (stage #3) which toggles the fan blower on/off will go off, which is no good as the pellets will go out if there is no air, please is it possible to make an add on to the circuit when (Stage #5) is activated the fan blower should be on steady!Also please can you specify the connections made between P1 , R18, R19, R20 really stuck at this point.

Thanks in advance
Vasilis K

My Reply:

Hi Vasilis,

Please do the following modification:

Disconnect D2 connection entirely, it's not required. That'a all, this will rectify the issue.

The circuit additionally requires a few corrections as suggested below:

1) Put a 10K resistor across pin#4 and ground of strage#5 IC, which is missing in the diagram.


2) The LDRs must be connected in parallel for better response and not in series as wrongly indicated in the diagram.


As regards the P1 setting, do it in the following manner:

initially keep the D1, R17 feedback disconnected.

Introduce the LDRs with the required amount of light from the burning pellets, and through some trial and error adjust P1 such that the output pin#6 of IC741 just becomes low or zero volts.

Now removing the light from the LDRs should make the output high or equal to the supply voltage, check this a few times to confirm the results.

That's all, the IC is all set.....now reconnect D1.R17 back in position.

Remember the LDR should not receive light from any external source, otherwise the whole circuit will malfunction.


I hope you got the points,

Regards. 

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