ELECTRONICS - [solder station protection] - [page 3/5]
3. SCHEMATIC DESIGN
:: 3.1 Power Supply
The circuit is powered by a transformer of 12VAC (input voltage is 230VAC/50 Hz, Belgium).
After the transformer, a bridge rectifier is used to convert the AC voltage to a DC voltage
(pulsating). C7 is used to reduce rimple, so a +/- flat DC voltage is present at the input
of the 7812 regulator. This regulator gives an output voltage of 12VDC, very stable. (The
capacitors C6 and C5 are used to reduce noise). The circuit asks no more than 50-100 mA,
won't dissipate too much power (heat). I've added a little heat sink anyway, just in case.
The input voltage of 230VAC is applied to connector J2, while the output voltage is available at connector J1. When the relays in the circuit are active, a voltage of 230VAC is present here.
:: 3.2 Timer/Trigger Button
The 555 and the components around it forms the actual timer. This timer is powered with a
stabilized 12VDC voltage. The time that the timer is active, can be set by resistor R6.
This is a trimmer, but could be replaced by a potentiometer. When the timer is active,
output pen 3 of the 555 is high, and makes transistor T1 conducting. Now the relay S1
closes and the solder station will be powered.
To start the timer, pin 2 of the 555 must be connected to the ground. To stop the timer, pin 4 of the 555 must be connected to the ground. I don't need the stop button, so no switch is foreseen between pin "4" and GND.
Like mentioned before, I only had a NC-button available. This type of button breaks contact when you press it. But it should be otherwise for the START-button, so the function of the button must be inverted. I've used a logic gate for that: the NAND (IC CD4011 in the CMOS series), but configured to use as a logical NOT gate. The output of this gate is always the inversion of the input: is input "high", then output is "low" and vice versa. This logic gate drives a transistor directly: when the output of the logic NAND is "0", then the transistor doesn't conduct: no start condition. But if the output is "1", the transistor conducts, and connects pin 4 with the ground (start condition). R9 is a pull-down resistor, and makes sure the input of the gate is never floating: CMOS gates must always be connected with either +V or GND.
Button pressed
Button not pressed
:: 3.3 Turn-off sound
A sound should be heard for a few seconds, after the timer was stopped again. So I started
thinking about a solution to do this. I already used the CD4011 in the circuit, where I
still got 3 ports available. I could use these.
NAND port 1 and 3 are placed in cascade, so they actually act as one AND port. This AND port compares the current state of the output transistor, with the previous state (held for a few seconds by the capacitors C4 and C3). If both inputs of the AND port are "1", then the buzzer will produce a sound. The duration of the sound, depends on the capacitors C4 and C3. During my tests, I had a sound for a few seconds. (Eventually you could use a much smaller capacitor, and use a second 555 to produce sound for a certain time).
Circuit is first powered
START is pressed
TIMER stops
:: 3.4 The complete diagram
The complete diagram can be viewed in a different Window.
It contains all the parts that were described in topic 3.1 - 3.3.
