Light and Beeper

Discussion in 'Electronic Basics' started by ZhangLu, Jun 20, 2004.

1. ZhangLuGuest

Dear all,

In my last posting, Terry Pinnell provided me the following circuit for a
device that when triggered on with a momentary N/O switch, it beeps and a
light will be on. When the switch is triggered again, the whole device will
be shut off.

The circuit is working fine except when the power is turned on, the buzzer
just buzz....until the N/O switch is triggered, the buzz then stops and
turns to beep and the bulb lights up. When the N/O switch is triggered
again, the light will be off but the buzzer will buzz on non stop.

Any suggestion to modify the circuit to eliminate the unwanted non stop
buzz?

Thanks.
ZhangLu
New Learner, Malaysia

2. Robert C MonsenGuest

I believe that your problem is that when the circuit starts up, it
lands in the wrong state. There is a 'race' condition on startup,
where both of the gates U2A and U2B have low input, and thus make
their output high. As both outputs are going up, the 4.7k resistor and
need to drive U2C should make it the case that U2B will lose; its
input will get high before the input to U2A, thus causing its output
to drop.

Since that is supposedly the right state, it seems like the circuit
should come up properly. I'm guessing that variations in parts,
perhaps, (or maybe my analysis is wrong!,) is causing it to come up in
the other state, meaning it'll buzz on startup.

You can fix it, though.

VCC
|
.-.
| | 1k
| |
'-'
| 4.7k
| ___
| +--------|___|----------------+
| | __ __ |
| +--| | |-| | | to oscillator
| | |& |o----+----+ |& |o------+-------------
+------|--|__| | |-|__|
| | .-.
| | | |
| | | | 220k
| | '-'
--- 1uF | |
--- | |
| +---o _/o---+
| / |
| --- 0.047uF
| ---
| |
+------------------+
GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

Get a 1k resistor, and a 1uF capacitor. Attach the 1k resistor to Vcc,
attach the resistor to the capacitor, and attach the other end of the
cap to ground. Then, connect one of the inputs of U2A to the junction
of the resistor and cap instead of what it is already connected to.
What this will do is ensure that U2A's output is low for a long while
after the power is turned on. That should ensure that the system comes
up in the state you care about, ie, the input to the oscillator LOW.

Another possibility is that the capacitance through the mosfet is
triggering the circuit when the 12V supply comes up. That seems
unlikely, though, since the capacitance is going to be less than 1 nF.
However, if its that, this modification proposed will fix it, since
U2A won't be able to trigger until the 1uF cap is charged up to the
logic transition point.

Regards
Bob Monsen

3. Terry PinnellGuest

I reckon it's something unique to your components, or construction, as
Bob Monsen suggests. It worked fine here when breadboarded. Hopefully
Bob's suggestion will fix it.

4. Terry PinnellGuest

BTW, I meant to add a request: could you try to stick to one thread in
one newsgroup please? It took me a while just now to trace the
several subjects.

5. ZhangLuGuest

Yes sir! Sorry for the inconveniences! Thanks for everything.

ZhangLu
Malaysia

6. Terry PinnellGuest

Incorrect. The circuit is for a DC buzzer, as the OP requested. So
'the last 2 gates' are not generating the tone. They constitute a
gated astable, as the schematic clearly shows.

7. JeffMGuest

could you try to stick to one thread in one newsgroup please?
I'll take part of the blame for that.
I chastised ZhangLu for posting to S.E.D. with a beginner's problem.

8. JamieGuest

from what your describing it seems that you are using an actual buzzer
on the output.
when you should be using a speaker cause the last 2 gates are actually
generating the tone.
if a buzzer is what you want to use then you can simplify the circuit
by simply connecting a fixed made buzzer to Q1 as the bulb is.

9. Rich GriseGuest

Yeah, and another word for astable is "oscillator." So why are you
oscillating the input to the buzzer?

Thanks,
Rich

10. Robert C MonsenGuest

Actually, I looked at it wrong (I, too, thought that the astable was
generating the tone...)

To me, it looks like it will come up in the state ZhangLu says, ie,
it'll buzz, and the light will be off. When he pushes the button, the
light will light, and it will start to periodically buzz.

That N-channel MOSFET will be on initially, because the junction of
U2C and U2D will be high when the astable is stopped.

I'd say you need to connect the gate of Q2 to the output of U2D...
that may fix it.

Regards,
Bob Monsen

11. Terry PinnellGuest

I screwed up the drawing, sorry. That's the second time I've mangled
it (the first was even worse, as pointed out in original thread by
Fred and Soeren). This time it was the CMOS chip number; it should be
a *4001* NOR, not the rare '4011 NOR' I called it before! The
corrected schematic is at
http://www.terrypin.dial.pipex.com/Images/LightBeeper-ZhangLu2.gif

As for the one I actually breadboarded a month ago, which I recall
worked reliably, I'm darned if I know whether that had 4001 or 4011.
It can't have been *simply* a matter of my drawing the wrong chip, as
this revised one needs a Low to gate the astable, not a High, and you
need to take the lamp driver from pins5/6 now, not pin 4, so that the
lamp is on permanently when beeping is occurring. BTW, it comes up
correctly OFF each time I tried it, with 3 different 4001 chips, i.e.
no beeping, no lamp.

Scrabbling around for excuses (premature senility apart), I blame Don
Lancaster. I was originally trying to build it with a 4093 Quad NAND
Schmitt. Don's diagram in his 'CMOS Cookbook' (page 126 of my 1977
copy), shows a *NOR* symbol for all the gates. All went downhill from
there <g>.

Zhanglu: Sorry for the slip. You can either use the revised circuit,
or if you went out and bought just a 4011 and don't have a 4001 let us
know and we should be able to redesign for that.

12. Terry PinnellGuest

Lazy post - you can do better!

Clue 1: Parts List describes BUZZ1 as 'DC Buzzer, e.g 12V'

Clue 2: What sort of frequency do you estimate you'd get with an RC of
2.2M and 4.7uF respectively?

Clue 3: My timing diagram here
http://www.terrypin.dial.pipex.com/Images/LightBeeper.gif
as posted in
Newsgroups: sci.electronics.basics,sci.electronics.design
Subject: Re: Looking for simple 555 relay circuit
Date: Tue, 25 May 2004 23:13:33 +0100
Message-ID: <>
and confirmed as correct by OP. Note 'Beep'. That's the output of a
gated astable (gated oscillator if you prefer). It's used to switch DC
current to a DC Buzzer.

--
Terry Pinnell
Hobbyist, West Sussex, UK

Clue 4: OP's quote "

13. ZhangLuGuest

Hi everybody,

Just to inform you that the device is working fine with 4001 NOR, the lamp
driver is taken from pins 5/6 .

I'll be working on some other new projects soon.