In one of my earlier posts I designed an interesting universal push button controller circuit which could be implemented with any related appliance for achieving a two-way push button control for the particular appliance.
We implement the same concept for the present application too.
Let's try to understand the above shown push-button heater controller circuit in detail:
The design can be divided into two main stages, the LM3915 stage which becomes responsible for creating an up/down sequentially varying resistances in response to the two push button's pressing, and the transistorized astable multivibrator stage which is positioned to respond to the varying resistances from the LM3915 outputs and generate a correspondingly varying PWMs. These PWMs are finally utilized for controlling the connected heater appliance.
You may be already knowing that the IC LM3915 is designed for producing a sequentially incrementing output across its pins 1 to 18 to 10, in response to an incrementing voltage level at its pin#5.
We take the advantage of this feature and employ a charging/discharging capacitor at its pin#5 via push buttons for implementing the required forward/reverse sequentially running logic low across the mentioned pinouts.
When SW1 is pushed ON, the 10uF capacitor slowly charges causing a rising potential at pin#5 of the IC which in turn enforces a jumping logic low from pin#1 towards pin#10.
The sequence stops as soon as the push button is released, now to force the sequence backwards SW2 is pressed which now begins discharging the capacitor, causing a reverse jumping of the logic low from pin#10 towards pin#1 of the IC.
The above action is indicated by the chasing red light across the relevant output pins in the same order.
However the actual implementation of the proposed push button controlled heater circuit is carried out by the introduction of the PNP transistor astable PWM generator circuit.
This astable circuit generates an approximately 50% duty cycle as long as the resistor capacitor values across the bases of the transistors are at an equilibrium, that is the values are equal and balanced, however if any of these components values are changed, a corresponding amount of change is introduced across the collectors of the devices, and the duty cycle changes at the same proportion.
We exploit this feature of the circuit and integrate one of the bases of the transistor with the sequencing outputs of the LM3915 via an array of calculated resistors which correspondingly change the base resistance of the concerned transistor in response to the pressing of SW1 or SW2.
The above action produces the required varying PWMs or duty cycles across the transistor collectors, which may seen hooked up with a triac and the heater appliance.
The varying PWMs enable the triac and the appliance to conduct or operate under the induced amount ON or OFF switching creating an equivalent amount of increase or decrease in the heat of the appliance.