Re: Wireless doorbell switch




BoB wrote:
> Thanks for that Terry, I am building a set of starting lights
> for mini moto racing, im using a 555 and a 4017 to give
> the "Christmas tree" effect. I just need a way of starting the count
from
> the start line, so I figured a wireless doorbell would be a cheap
> option, Wilkos sell them for under £5.
> Im planning to hook the output up to the reset pin of the 4017 to
> start the sequence. Maybe leave the first couple of outputs unused
> to allow for any false starts due to the chime pattern of the
doorbell.
> Hope Im making sense now, I realise my first question was a little
vague.
>
> Bob

Hi, Bob. There have been a few questions recently about using these
cheapie wireless doorbells for remote control. I happened to have one
around, so I took it apart to see what's inside. And if yours is like
mine, you can do your circuit fairly easily.

I've got a "Dimango by Lamson Home Products", Model 3110R. The
receiver is powered by 2 AA batteries, which gives 3VDC. There is a
coil and some discretes and transistors for the analog front end, with
a CMOS 4069 in the front end, too. This goes to a C.O.B. (Chip on
Board) PIC or other cheapie microcontroller (uC) mounted on a separate
small circuit board soldered to the main board. As far as I can see,
there's only one output from the uC -- the one going to the dinger.
That's a small 8 ohm 1/4 watt speaker.

When you're looking at interface, many times you don't have to have
Yoda-like understanding of the entire circuit -- just the part that
you're working with. I tracked down the circuit board traces and came
up with this for the output (view in fixed font or Notepad):

` VCC
` +
` |
` |
` | __ /|
` '--| | |
` .--|__| |
` | \|
` | 8 ohm
` COB ___ |/ 1/4 watt
` o-----|___|--|
` Output R |>
` |
` |
` ===
` GND
(created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de)

Well, that's something we can use. You know that with a 3V power
source, you'll probably get at least 2V square wave across the speaker
when the doorbell output transistor is on (transistor saturation and/or
battery voltage drop will keep the transistor from going fully on).
Now we're getting somewhere. We know it won't hurt the anything to
replace the speaker with a 1K resistive load like it shows in Mr.
Pinnell's link (you don't need the bell sound, anyway), and we can use
that voltage drop across the resistor to interface to the digital stuff
with another transistor:


VCC
+
|
.-.
| |
1K| |
'-'
|
|
o--->
| Logic Level Signal
|
|
|
|
___ |/ 2N
o----o------|___|--| 3904
From | 3.3K |>
Door .-. |
Bell | | |
Spkr | |1K |
Drive '-' |
| |
o----o ===
| GND
===
GND
(created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de)

At this point, if you can program a PIC, you're almost home. You can
ignore the several seconds of pulses which follow the start of the
doorbell ring, and start your sequence. Since you can get 20mA out of
a single output pin as long as you don't exceed 40mA per port, you can
drive the LEDs directly (with a current-limiting resistor), with one
input pin and 6 output LEDs (5 amber, one green). If you wanted to be
really crafty, you could just replace the speaker with a 100 ohm
resistor, use the two battery power supply for the PIC and forget the
level-shifting transistor.

But your post suggests you want to do this with 555s and a 4017, which
leads me to believe you don't have that capability. Doing this with
digital logic ICs, you're going to have some issues. The biggest one
is that the transistor will be turning the speaker on and off for
several seconds. It's a square wave, right? You want to have a single
pulse which starts when you press the button. So, you can use the
transistor pulse to trigger a 555 to a single pulse longer than the
time the doorbell is sounding (let's be generous and say 10 seconds).


VCC VCC VCC VCC
+ + + +
| | | |
.-. .-. | |
| | | |150K | |
1K| | | | | |
'-' '-' | |
| | .---o-----o---.
| | | 8 4 |
o----------)----o 2 |
| | | | A
| | | 3 o----->
| o----o 6 |
| | | LM555 |
| | | |
___ |/ 2N o----o 7 |
o----o------|___|--| 3904 +| | |
From | 3.3K |> --- | |
Door .-. | 100uF --- | 1 5 |
Bell | | | | '---o-----o---'
Spkr | |1K | | |
Drive '-' | | |
| | | |
o----o === === ===
| GND GND GND
===
GND
(created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de)

Now you have to look at some details. Your 555 has a positive-going
pulse when it's turned on, but the 4017 requires an active low (the
reset input to the 4017 is valid for a "1" input). So, you can use
another transistor to invert the logic. You need another 555 to count
down the Christmas Tree, and you can use the terminal count ("6" below)
to put CLKEN (clock enable) high, which inhibits counting. That will
mean the green LED will stay on until the first 555 goes off. Here's
The rest of the circuit (again, view in fixed font or Notepad):


VCC
+
A >---------------o--------. |
| | .-.
VCC | | | |4.7K
+ | | | |
| | .-. '-'
.-. VCC | 47K| | |
R1| | + | | | o-----.
| | | | '-' | |
'-' .---o----o---. | |/ |
| | 8 4 | '----| |
| | | |> |
o-----o 7 | | | VCC
| | | | | + .----------.
.-. | | === | | | |
R2| | | 555 | GND | | | |
| | .--o 6 | | | | |
'-' | | | .--------o-----o------o-------. |
| | | | | RST Vdd CLKEN | |
o--o--o 2 3 o--------o CLK | |
+| | | | 4017 | |
C --- | 1 5 | | | |
--- '---o----o---' .--oVss | |
| | N.C. | | "0" "1" "2" "3" "4" "5" "6" | |
=== === | '--o---o---o---o---o---o---o--' |
GND GND === | | | | | | |
GND | | | | | o-----'
| | | | | |
10K.-. .-. .-. .-. .-. .-.
all| | | | | | | | | | | |
| | | | | | | | | | | |
'-' '-' '-' '-' '-' '-'
| | | | | |
v v v v v v



VCC
+
|
|
V ~ Amber
- ~(Green)
|
|
.-.
| |
| |R5
'-'
|
From 4017 & |
10K Res. |/
>---o----| 2N3904
| |>
.-. |
10K| | |
| | |
'-' |
| |
=== ===
GND GND
(created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de)

Note that each of the 4017 outputs also goes to a transistor as shown,
with the terminal "6" driving a green LED instead of an amber one.
I've left the choice of resistor values to you depending on what kind
of power supply you use and what kind of timing you'd like. For a 5VDC
supply and 1 second Christmas Tree timing, you might want to try 470K
for R1 and R2, 10uF for C, and 220 ohms for the 6 ea. R5. I'd
recommend against using the 3VDC of the receiver for the rest of the
circuit -- it won't interface properly here -- use the external,
regulated supply.

So, about 1 second after you press the doorbell transmitter, the amber
"1" will light up, 1 second later the "2" will light up, and so on. 5
seconds after doorbell press, the green "6" will light up, and stay on
for however long the 1st 555 is on (about 10 seconds above). That's
because the "6" is also driving CLKEN. As long as that's low, the 4017
will clock. When it goes high, the 4017 will stop clocking no matter
what's going on at CLK. But after 10 seconds, the 1st 555 will go off.
At that time, the 4017 will be forced into reset and all the lights
will go off until you press the transmitter button again.

If you don't want to use LEDs, you might want to use the transistor
outputs to drive other transistors, small relays, or use logic level
TO-92 triacs to drive a low voltage AC bulb. Your call.

Try this circuit out as it stands, though, and see if it works. Feel
free to post again if you've got other questions.

Good luck
Chris

.