Design Philosophy

Some people seem to be afraid of doorbells and just stab at the button hardly giving the bell or chime a chance to
register. Others stick their finger on it and hold it there, while yet others repeatedly stab at it. This design deals
with all such problems by giving the bell/chime a fixed 'on' time as well as a minimum time before it can be activated
again.


Circuit Description

In the rest state U1a-d supplies current via R5 to D1. Four gates are wired in parallel to provide sufficient current
for D1 without stressing the chip outputs. When the bell push is pressed a 2V step is generated which is turned into a
short duration pulse by C1/R1/R2. This is the first line of defence. It doesn't matter how long someone holds the button
you only get a short pulse here.

Q1 uses this pulse to very rapidly charge C2 via R6 which reaches the full supply voltage before the pulse disappears.
This voltage is applied to U1f whos output goes to logic 0 and now similarly charges C3 via D2/R9. At the same time Q2
is switched on by R10, powering the bell/chime.

Charging C3 sets U1e output to logic 1, so U1a-d outputs go to logic 0. This now means that no current is available for
D1 ensuring that there absolutely no possibility of another pulse being generated until the cycle completes.

R6 is used to limit the charge current to C2, protecting Q1. It also ensures charging of C2 doesn't slow down the
initial switching of U1f. R9 performs a similar function for U1f/C3/U1e. The very rapid switching of this circuit
provides effective de-bounce for the bell push as once triggered, R5 holds C1 even if the bell push breaks contact.

After a while C2 is discharged by R3/R4 to the point where the schmitt action of U1f takes place and the output goes to
logic 1. Q2 now switches off, so a chime gives it's release tone (or a bell stops sounding).

D2 prevents U1f or R10 affecting the discharge of C3, which now takes place via R7/R8/R9. Once the schmitt action of U1e
occurs the outputs of U1a-d go to logic 1 again, and power is restored to the LED so that another press of the bell push
can be registered. If the button is still being held at this time, nothing will happen until it is released.

As well as giving a visible indication, D1 helps ensure a low impedance path so that spurious EMI can't falsely trigger
the circuit, and damp won't upset it either. Conversely the impedance is high enough so that a very long cable run, or
bad contacts on the bell push won't interfere with operation either.

The unit will reliably work with a supply as low as 4V. If you want to use higher voltages you should increase the value
of R5 to limit the current demanded from U1a-d. Under the conditions shown it is about 20mA. You shouldn't try to draw
more than 25mA. The absolute maximum voltage is 15V.

Most bells and chimes draw between 200mA and 1A. The unit can drive up to 3A from a not especially 'clean' 6V source.

If you use this design to drive a buzzer, I suggest an additional capacitor (around 100n) across the terminals of the
buzzer itself for EMI suppression.


Component Choices

D1 is intended to be fitted inside the bell push to give visitors positive indication that it is working. Many
pushbutton housings are translucent which is ideal, as the LED is then fully protected. Otherwise a small hole should be
drilled and the diode glued in place.

Q1 really does need to be a C grade transistor to ensure complete charge of C2 in the short pulse time.

With the values shown for C2/C3 'time to dong' can be set between about 200mS and 2S by R4, and 'time before next ding
allowed' range is about 1S to 10S, set by R7. If you increase C2 for an even longer 'on' time you will probably need to
increase C1 as well to ensure a full charge. Increasing the value of C3 doesn't have the same problem, as it can take
the whole of the 'on' time to charge.

I used skeleton presets for R4 and R7 but you could use spindled pots if you like, with knobs for external adjustment.


W Godfrey 1979-2006