Assembled A 555 Based Timer For Water Boiler
We have an assembled water boiler in our house. This was made many years back, when I was working in BPL and staying in Bangalore. A stainless steel vessel was fit with a heater and thermostat at the bottom. The minimum temperature has to be set at around 35◦C or more without which the thermostat did not function, which was ok for us for the climate at Bangalore. But when we returned to our native place in Kerala, the climate of which is very humid and warmer, the temperate of minimum 35◦C was too much.
Though I tried using different types of temperature controllers, all of which worked on metal expansion/contraction basis, it was not shutting off and many times, the water reached the boiling stage causing very high electricity consumption and wastage of hot water. That is how I bumped upon the idea of fixing a timer. From my experience, keeping the heater on for about 12-15 minutes was enough to reach the temperature required for our use. So I used the following timer board for assembling a control circuit through a 30A contractor relay, a hand-drawn schematic of which is given below:
I studied the following link in detail and modified the above board to replace the capacitor to 1000/63V and the trim pot to 1 Meg. The rest of the schematic of the 555 Timer shown below will not match with the above board, which is ready to buy type from the open market. (I had stock of these boards which I bought for some other project)
I adjusted the timer to switch on the relay after about 15 minutes, which I did by applying power to the board and adjusting the resistance to about 812K as mentioned in the article. I had to readjust the trim pot after a couple of trials to reach this level. Then next task was to obtain the following parts for assembling the timer:
- 30A Contractor – two pole, coil working on 230V AC directly. (The AC inputs are given to L1 and L2 and Load is given to T1 and T2. When the coil gets energised, the connection gets closed between L1 to T1 and L2 to T2)
- Casing for the contractor
- Transformer to make power supply for the 12V 555 Timer board. (I used a 7.5V-0-7.5V, 500mA which I had in stock and connected both ends without using the centre tapping. This provided about 15V AC)
- Bridge rectifier – 1 No.
- 0.01uF/100V caps – 2 Nos.
- 2200uF/25V cap – 1 No.
- 220uF/25V cap – 1 No.
- 7812 regulator IC – 1 No.
- Fuse holder with Fuse of 1A (for primary of transformer)
- LEDs – 2 (One Red and One multicolor)
- IN4007 diodes– 2 Nos.
- 1/4W 1 Meg resistor – 2 Nos.
- Inter connecting wires.
As the 230V AC coil was drawing only around 59mA current, power consumption even if it was left on was very negligible and further the relay in the timer could take the load very easily:
First I fixed the contractor in the base of the case I bought for it. Then fixed the transformer on its side. Soldered the ground point of bridge rectifier to the metal case of transformer. Then soldered the two 7.5V AC inputs to it. Soldered the ground point of 2200uF cap to the transformer and positive to the + of bridge rectifier. Soldered 0.01 caps across the two AC in/+ and – points of bridge Rectifier. Then soldered the 7812 regulator heat zinc to the metal case of transformer, which was the ground of it. Connected the + output of the bridge rectifier to the input of 7812. Soldered the 220uF to the output pin of the 7812. Fixed the timer board on the other side of the contractor and wired the 12V power supply. L1 of the contractor was earmarked for Phase (Line) and L2 for Neutral. My requirement was to switch off the circuit after a delay of about 15 minutes. So the coil of the contractor was connected to the NC (normally closed) contact of the timer relay and the other end of the coil was connected to L2 (Neutral). Wired the 230V AC input for the transformer to the L1 and L2 of contractor, through a 1A fuse, holder of which was fixed on the front cover. Then L1 (phase) was wired to the single pole of the Timer relay. The idea was that the coil will remain energized pulling the plunger for closing the contacts between L1 to T1 and L2 to T2, until the relay gets on after the delay, upon which the AC input to contractor will be cut, releasing the plunger. (I am sure many of you might know that the contractor works on solenoid principle.
When the coil is energized the electromagnet pulls the plunger and holds it tight and releases it when cut.) Red LED was fixed on the cover, one side of which was connected to L1 through a 1Meg resistor. The other end was connected to L2 through one IN4007. Did the same for the T1 and T2 also using the colour LED fixed on the cover. The idea was that the colour LED will emit different colours catching the eye when the heater is on. It will go off when the contractor is shut off by the timer. The timer board itself had a red SMD LED that remains lit on power on. A blue LED will glow when the relay is switched on. This was found useful, as glow from both these SMD LEDs were visible through the case of the Contractor. Now let us take a look on the work done:
I applied power to the L1 and L2 and tested the unit on my worktable and found it to be working perfectly well, for which I had to leave it on for about 15 minutes. Then fixed the unit on top of the switch board of the heater. Opened the switch board and connected the output (Line and Neutral) of the two pole AC switch to L1 and L2 and connected the heater load to T1 and T2. Unfortunately, the two pole switch broke down and caused a temporary halt to my installation. I had already switched off the power to this line. So, I removed the broken switch and went to market and really hunted for getting an exact replacement, but in vain. So, I had to buy a new switch, size of which was different from what I carried along with a new case, which I fixed on top of the existing switch board. The timer worked very well and water got heated up enough for direct use without a need to mix cold water. I watched its performance like a director watching a trial run after completion of his work. Now we can save electricity and be carefree!
Mission accomplished and satisfaction intensified jumped into its collection bag!
This article was prepared for you by Parasuraman Subramanian from India. He is 72 years old and has more than 30 years’ experience in handling antique equipment like Valve Radio, Amps, Reel Tape Recorders and currently studying latest tech-classes conducted by Kerala State Electronics Technicians’ Association. He has done graduation in BBA degree, private diploma in Radio Engineering and retired as MD of a USA company. Presently working as Consultant to Hospital and other institutions.
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