Re: NEED HELP! Simple DC circuit

On Mar 14, 10:04 pm, ehsjr <e...@xxxxxxxxxxxxxxxx> wrote:
DaveK wrote:
What do I need? 555/556 Timer? Flip-Flop? I have a basic
understanding of electronics, but most likely would not be able to set
up this circuit without a schematic. At this point in my education,
telling me what to do, without a schematic, probably wouldn't help me.
Believe me I'm trying as hard as I can to learn this stuff. I'm
really getting hooked on it, but I'm not even close. I've played with
the timers but that's about it. This circuit doesn't seem that it
would be very hard for someone who knows what they are doing, but then
again what do I know? Thank you for your time and any help would be
greatly appreciated.

I have a 4.5vdc (battery operated) circuit with three momentary
switches. The 1st two switches are in series and when closed power a
buzzer for the amount of time that they are both closed. The third
switch when closed powers a LED. What I want to happen is when the
third switch is activated, but not continually closed (momentary
switch) it will trigger power to the LED and continue to stay on till
one or both of the 1st and 2cd switches are closed once again. The
third switch may never need to be on, but if it does it will need to
stay on till one of the other two is closed.
TThank you for your time and any help would be greatly appreciated.

Here's a circuit that will do what you described. I
assumed a relatively low current requirement for
the buzzer.

PNP 2N3906
+5 -+--- ----------[1K]--------+
| e\ /c |
| --- [LED]
| | |
| | [SCR1] P0118MA2AL3
| | Pb3 /|
| | ___ | |
`----------+---o o---[47K]---+ +-----------+
| | |
| | Pb2 |
| | ___ |
| +---o o---[Buzzer]-----+ |
| | | | |
| | +--->|-----+ | |
| | D2 | | |
| | Pb1 | | |
| | ___ | /c |
| +---o o---[1K]---|---| NPN |
| | | \e |
| +--->|-----+ |2N2222 |
| D3 | | |
| D1 | | |
`--->|---+---[.1uF]---+ | |
| | | |
| | | |
[1K] [10K] | |
| | | |
Gnd ----------------+------------+-----+--------+

Pressing Pb3 turns on the SCR, which stays on, lighting the
LED, until current through it is interrupted. The PNP is
biased on through D1 and the 1K resistor. When either PB1
or PB2 is pressed, a pulse is generated through the .1 uf
capacitor, which briefly biases D1 and the PNP off,
interrupting the current through the SCR, so it turns off.
An SCR (Silicon Controlled Rectifier) is a device that
once turned on by an external signal at its gate, stays on
even though the external signal goes away, until current
through it is interrupted.

Pb1 turns the NPN on while it is pressed, connecting one
side of the buzzer to (-). If Pb2 is also pressed, the other
side of the buzzer is connected to (+), and it sounds.

The SCR is mouser part # 511-P0118MA2AL3, and the diodes
are 1N4148 or 1N914 If the buzzer draws more than about 200 mA,
substitue a relay for the buzzer in the circuit, and use the
relay contacts to complete the circuit to the buzzer. If this
is a "hardware store" variety buzzer, you'll need the relay with
a 1N4148 diode across it backwards, and will need to add a 1N4004
diode across the buzzer too (also backwards). "Backwards" means
the cathode end (the end with the stripe) connects to (+) instead
of (-).

Ed- Hide quoted text -

- Show quoted text -


You might need a smaller resistor in series with the LED. I think the
SCR holding current is around 5mA and you only have about 1 mA in the
LED. The SCR will drop 1.5 or more, and the PNP will drop another .5
and the LED drops 1.8, so you only have 5-(3.8) = 1.2 volts across the
resistor, or 1.2 mA.

-Bill


.

Relevant Pages

  • Re: NEED HELP! Simple DC circuit
    ... up this circuit without a schematic. ... buzzer for the amount of time that they are both closed. ... switch when closed powers a LED. ... Pressing Pb3 turns on the SCR, which stays on, lighting the ...
    (sci.electronics.basics)
  • Re: NEED HELP! Simple DC circuit
    ... up this circuit without a schematic. ... buzzer for the amount of time that they are both closed. ... switch when closed powers a LED. ... Pressing Pb3 turns on the SCR, which stays on, lighting the ...
    (sci.electronics.basics)
  • Re: Ladder diagram to CMOS logic
    ... push button switch. ... I would like to sound the buzzer "x" when the input is low and when ... the button is pressed the relay would be switched on and holds itself ... I would like to convert this into a circuit using cmos chips. ...
    (sci.electronics.basics)
  • Converting Ladder to cmos logic
    ... switch. ... I would like to sound the buzzer "x" when the input is low and when ... pressed the relay would be switched on and holds itself through the ... I would like to convert this into a circuit using cmos chips. ...
    (sci.electronics.basics)
  • Converting Ladder to cmos logic
    ... push button switch. ... I would like to sound the buzzer "x" when the input is low and when ... the button is pressed the relay would be switched on and holds itself ... I would like to convert this into a circuit using cmos chips. ...
    (sci.electronics.basics)