Re: Circuit Design - Button activated timer with small "chirper"?

HighFlight wrote:
Hello all. I have a personal need for a small circuit. I have a good understanding of components, how they are connected, and actually building them, but an electrical engineer I am not. I was wondering if someone could lend me a hand. It's probably very simple in design. Attached are my requirements:

* Needs to be as small and light as possible.
* Attachable with Velcro. I'm imagining a PCB where the back has a stick on Velcro pad.
* Small momentary button that when pressed, activates a 15 second timer (approximately and no display needed). When the 15 seconds expires, a small speaker should emit an audible chirping sound (audible from within approximately 50-100 feet outdoors in a relatively quiet setting) at 5 second intervals. This chirping will continue until the button is pressed again.
* There may be two buttons if size/weight/complexity is smaller/lighter/simpler than required circuitry with single button.
* Replaceable battery (I'm imagining a watch size battery).
* Should be able to take a bit of bumping around. It is for a child's use and will be attached to something that will undoubtedly be thrown or dropped.
* Did I mention as light as possible??? :-)

I've bought from Allied et all in the past, so buying the speaker, buttons, battery holder, etc. shouldn't be a problem. The physical layout and PCB building should be something I can do. This is just a little project for my son and me so I can just etch a board (hopefully one layer is enough). I just have no idea what components to use.

Thanks very much for any assistance!

Jon




The light weight requirement and 50-100' range is gonna
kill your battery life. I think the first thing you need
to do is find a sounder thatmeets your requirement for
audibility at the range you want, and then design from
there. The louder it needs to be, the more power it
will take from the battery. I think a Sonalert may give
you the best audibility vs power.

I'll ignore battery life for the following conceptual
level circuit. (You can Google for 555 circuits) It
requires 2 555's and a 4017, 2 mosfets and an SCR,
plus support components:

-----
/ | 555 |
Gnd ---o o---2| 15 |---A
Switch | secs|
-----

The 555 is configuired as a monostable.
Each press of the switch creates a + pulse
at point A for 15 seconds.

+----------+
| d|_
| | | P channel Mosfet
| |<| |
| s+-| |g___ to point B on 4017
| |
--- ------
A-+---|SCR| | 555 |
| --- | 5 |-----+
| | | secs | |
| | ------ |
| | | _|d
| +----------+ | | P channel mosfet
| | | |>|
+-->|-------+-----+___| |-|s
D1 | | |
| | |
| [1K] [Sounder]
| | |
Gnd ----+-----+-------------+

The 15 second pulse at point a is connected to the
gate of an SCR, which turns the SCR on, as long
as Q1 conducts. Q1 conducts while B is negative
(more on this later). That allows the 5 second 555
astable to run.

So, up to this point: 1 press of the button triggers
the 15 second time to run for 1 cycle, then stop.
That timer turns on an SCR, which will stay on as
long as current is drawn through it. Current is drawn
by the 5 second timer through Q1, so the SCR stays
on and the 5 second stable will keep running until
Q1 is turned off.

The output of the 555 astable feeds the sounder through
Q2. Q2 is held off during the 15 seconds that point A is
plus. Therefore, the circuit will not start sounding
during the 15 second cycle. After that, it will sound
every 5 seconds.

Now, all we need to do is control point B to be negative
until the second press of the switch. You can use a
flip flop or a counter to do that. I'll discuss using
a 4017 counter:
-----
A----|14 |
| 2|---B
| |
| 4|--+
| | |
| 15|--+
-----
4017

The clock signal comes in on pin 14, which is
connected to the output (point A) on the 15 second
timer. Each press of the switch triggers the 15
second timer. Each clock pulse advances the
4017 count by 1, until the 4017 is reset, at which
time the count returns to 0. As configured,
count 2 (pin 4) is connected to the reset pin
on the counter (pin 15). Therefore the counter
can only count 0,1,0,1,0,1 etc. That means that each
press of the switch changes the polarity on pin 2
(point B) from + to - or - to +

Ed
.

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