Specific 555 circuit

Henry · Dec 25, 2005

Hi, dabbling with some TTL stuff that I dug out of storage. Im about 25
years rusty since I tinkered with this stuff all those years ago. Anyway,
working on a project where it would be deal to have a 555-timer based
monostable multivibrator (with retrigger ability.) Is that possible with the
555? I have found many circuits online (actually mostly the same one over
and over again) but they dont seem to support re-triggering with the 555.

Henry

Henry · Dec 26, 2005

"Jasen Betts" <jasen@free.net.nospam.nz> wrote in message
news:6179.43aef3d9.e5e37@clunker.homenet...

On 2005-12-25, Henry <henrytj@pghmail.com> wrote:
Hi, dabbling with some TTL stuff that I dug out of storage. Im about 25
years rusty since I tinkered with this stuff all those years ago.
Anyway,
working on a project where it would be deal to have a 555-timer based
monostable multivibrator (with retrigger ability.) Is that possible with
the
555? I have found many circuits online (actually mostly the same one
over
and over again) but they dont seem to support re-triggering with the
555.

if by retrigerable monostable you mean have it restart its timing interval
while in the "on" state, no that can't be done using the 555 alone.

the 555 operates by charging (or discharging) a capacitor. to restart the
interval you'd have to arrange for the capacitor to be discharged back to
its starting voltage.

Bye.
Jasen

Okay, thanks. That foils my one idea. I have an old SN74123 Dual
re-trigerable monostable multivibrator. I had simply hoped that a single 555
timer could do the same so that I did not have to waste half the 74123
package. I only have a couple of these and no funds to acquire anymore of
them at the present.

Henry

John Fields · Dec 26, 2005

On Mon, 26 Dec 2005 16:19:06 -0500, "Henry" <henrytj@pghmail.com>
wrote:

"John Fields" <jfields@austininstruments.com> wrote in message
newsj30r11ode6lck5uhcvpb2b0tkcdnd6fc1@4ax.com...
On Sun, 25 Dec 2005 05:55:22 -0500, "Henry" <henrytj@pghmail.com
wrote:

Hi, dabbling with some TTL stuff that I dug out of storage. Im about 25
years rusty since I tinkered with this stuff all those years ago. Anyway,
working on a project where it would be deal to have a 555-timer based
monostable multivibrator (with retrigger ability.) Is that possible with
the
555? I have found many circuits online (actually mostly the same one over
and over again) but they dont seem to support re-triggering with the 555.

---

Try this: (view in Courier)

+V-----------------------+--------------+-------+
| | |8
[1K] [Rt] +---+---+
| | 2|_ Vcc |3
+--------------|--O|T OUT|--> OUT
| | 6| |
C +---|TH |
IN-------+-[0.1ľF]--+--B NPN | 7|_ 7555|
| | E +-----+--O|D |
| | | | +| | GND |
| | | D [Ct] +---+---+
[10K] +-----------G NCH | |1
| | | S | |
| [10K] | | | |
| | | | | |
GND>-----+----------+----+--------+-----+-------+

The input is a positive-going pulse or edge.

--
John Fields
Professional Circuit Designer

Thanks, I will give it a try. One question. I am uncertain what the NCH
component is. I assume its not hydrogen cyanide as that would be HCN.
---

It's a nitrogen carbon hydride switch.

Actually, it's an N-Channel enhancement mode MOSFET with a channel
resistance which will discharge Ct sufficiently close to ground
before the input pulse goes away so that you can predict the
increased length of the output pulse. If you don't have any
MOSFETS, an NPN transistor will work if Ct isn't too big.

--
John Fields
Professional Circuit Designer

Jasen Betts · Dec 25, 2005

On 2005-12-25, Henry <henrytj@pghmail.com> wrote:

Hi, dabbling with some TTL stuff that I dug out of storage. Im about 25
years rusty since I tinkered with this stuff all those years ago. Anyway,
working on a project where it would be deal to have a 555-timer based
monostable multivibrator (with retrigger ability.) Is that possible with the
555? I have found many circuits online (actually mostly the same one over
and over again) but they dont seem to support re-triggering with the 555.

if by retrigerable monostable you mean have it restart its timing interval
while in the "on" state, no that can't be done using the 555 alone.

the 555 operates by charging (or discharging) a capacitor. to restart the
interval you'd have to arrange for the capacitor to be discharged back to
its starting voltage.

Bye.
Jasen

John Fields · Dec 26, 2005

On Sun, 25 Dec 2005 05:55:22 -0500, "Henry" <henrytj@pghmail.com>
wrote:

Hi, dabbling with some TTL stuff that I dug out of storage. Im about 25
years rusty since I tinkered with this stuff all those years ago. Anyway,
working on a project where it would be deal to have a 555-timer based
monostable multivibrator (with retrigger ability.) Is that possible with the
555? I have found many circuits online (actually mostly the same one over
and over again) but they dont seem to support re-triggering with the 555.

---

Try this: (view in Courier)

+V-----------------------+--------------+-------+
| | |8
[1K] [Rt] +---+---+
| | 2|_ Vcc |3
+--------------|--O|T OUT|--> OUT
| | 6| |
C +---|TH |
IN-------+-[0.1ľF]--+--B NPN | 7|_ 7555|
| | E +-----+--O|D |
| | | | +| | GND |
| | | D [Ct] +---+---+
[10K] +-----------G NCH | |1
| | | S | |
| [10K] | | | |
| | | | | |
GND>-----+----------+----+--------+-----+-------+

The input is a positive-going pulse or edge.

--
John Fields
Professional Circuit Designer

Henry · Dec 26, 2005

"John Fields" <jfields@austininstruments.com> wrote in message
news

j30r11ode6lck5uhcvpb2b0tkcdnd6fc1@4ax.com...

On Sun, 25 Dec 2005 05:55:22 -0500, "Henry" <henrytj@pghmail.com
wrote:

Hi, dabbling with some TTL stuff that I dug out of storage. Im about 25
years rusty since I tinkered with this stuff all those years ago. Anyway,
working on a project where it would be deal to have a 555-timer based
monostable multivibrator (with retrigger ability.) Is that possible with
the
555? I have found many circuits online (actually mostly the same one over
and over again) but they dont seem to support re-triggering with the 555.

---

Try this: (view in Courier)

+V-----------------------+--------------+-------+
| | |8
[1K] [Rt] +---+---+
| | 2|_ Vcc |3
+--------------|--O|T OUT|--> OUT
| | 6| |
C +---|TH |
IN-------+-[0.1ľF]--+--B NPN | 7|_ 7555|
| | E +-----+--O|D |
| | | | +| | GND |
| | | D [Ct] +---+---+
[10K] +-----------G NCH | |1
| | | S | |
| [10K] | | | |
| | | | | |
GND>-----+----------+----+--------+-----+-------+

The input is a positive-going pulse or edge.

--
John Fields
Professional Circuit Designer

Thanks, I will give it a try. One question. I am uncertain what the NCH
component is. I assume its not hydrogen cyanide as that would be HCN.

Henry

John Fields · Dec 27, 2005

On Mon, 26 Dec 2005 16:31:40 -0600, John Fields
<jfields@austininstruments.com> wrote:

On Mon, 26 Dec 2005 16:19:06 -0500, "Henry" <henrytj@pghmail.com
wrote:

"John Fields" <jfields@austininstruments.com> wrote in message
newsj30r11ode6lck5uhcvpb2b0tkcdnd6fc1@4ax.com...
On Sun, 25 Dec 2005 05:55:22 -0500, "Henry" <henrytj@pghmail.com
wrote:

Hi, dabbling with some TTL stuff that I dug out of storage. Im about 25
years rusty since I tinkered with this stuff all those years ago. Anyway,
working on a project where it would be deal to have a 555-timer based
monostable multivibrator (with retrigger ability.) Is that possible with
the
555? I have found many circuits online (actually mostly the same one over
and over again) but they dont seem to support re-triggering with the 555.

---

Try this: (view in Courier)

+V-----------------------+--------------+-------+
| | |8
[1K] [Rt] +---+---+
| | 2|_ Vcc |3
+--------------|--O|T OUT|--> OUT
| | 6| |
C +---|TH |
IN-------+-[0.1ľF]--+--B NPN | 7|_ 7555|
| | E +-----+--O|D |
| | | | +| | GND |
| | | D [Ct] +---+---+
[10K] +-----------G NCH | |1
| | | S | |
| [10K] | | | |
| | | | | |
GND>-----+----------+----+--------+-----+-------+

The input is a positive-going pulse or edge.

--
John Fields
Professional Circuit Designer

Thanks, I will give it a try. One question. I am uncertain what the NCH
component is. I assume its not hydrogen cyanide as that would be HCN.
---
It's a nitrogen carbon hydride switch.

Actually, it's an N-Channel enhancement mode MOSFET with a channel
resistance which will discharge Ct sufficiently close to ground
before the input pulse goes away so that you can predict the
increased length of the output pulse. If you don't have any
MOSFETS, an NPN transistor will work if Ct isn't too big.

---
Aarghhh!!!

There's a gross error in the circuit which I'll fix tomorrow...

Hint: Vbe of the NPN won't allow the MOSFET to turn on, duh...

--
John Fields
Professional Circuit Designer

John Fields · Dec 27, 2005

On Mon, 26 Dec 2005 18:15:48 -0600, John Fields
<jfields@austininstruments.com> wrote:

Aarghhh!!!

There's a gross error in the circuit which I'll fix tomorrow...

Hint: Vbe of the NPN won't allow the MOSFET to turn on, duh...

---
KISS works best...

+V
|
+-------+
| |8
[Rt] +---+---+
| 2|_ Vcc |3
IN>---+----------|--O|T OUT|--> OUT
| | 6| |
| +---|TH |
[1000] | 7|_ 7555|
| +-----+--O|D |
| | +| | GND |
| E [Ct] +---+---+
+--B PNP | |1
C | |
| | |
+-----+-------+
|
GND

Unlike the vanilla 7555 which starts timing out when pin 2 goes low,
with the retrigger it starts timing out when pin 2 goes high. That
means that pin 3 will be high for the timeout period (1.4RC) plus as
long as pin 2 stays low:

_
t = (1.4RC) + (tT)

If you don't already have it you might want to download Linear's
excellent simulator at:

http://www.linear.com/company/software.jsp

It's free, and you can use it to build stuff without having to buy
parts, which might be handy for you in your present situation.

Anyway, here's the netlist for the circuit above if you want to
check it out:

Version 4
SHEET 1 960 680
WIRE -432 352 -432 224
WIRE -432 528 -432 432
WIRE -256 336 -256 288
WIRE -256 528 -432 528
WIRE -256 528 -256 416
WIRE -144 224 -432 224
WIRE -144 432 -144 224
WIRE -80 352 -80 80
WIRE -16 528 -256 528
WIRE -16 528 -16 160
WIRE 48 160 -16 160
WIRE 48 224 -144 224
WIRE 48 288 -256 288
WIRE 48 352 -80 352
WIRE 64 432 -144 432
WIRE 272 432 144 432
WIRE 336 80 -80 80
WIRE 336 160 272 160
WIRE 336 160 336 80
WIRE 336 224 272 224
WIRE 336 288 272 288
WIRE 336 288 336 224
WIRE 336 320 336 288
WIRE 336 384 336 320
WIRE 336 528 -16 528
WIRE 336 528 336 480
WIRE 432 160 336 160
WIRE 432 192 432 160
WIRE 432 320 336 320
WIRE 432 320 432 272
WIRE 432 352 432 320
WIRE 432 528 336 528
WIRE 432 528 432 416
WIRE 656 160 432 160
WIRE 656 240 656 160
WIRE 656 528 432 528
WIRE 656 528 656 320
WIRE 656 576 656 528
FLAG 656 576 0
SYMBOL Misc\\NE555 160 256 R0
SYMATTR InstName U1
SYMBOL voltage -432 336 R0
WINDOW 3 24 104 Invisible 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR Value PULSE(5 0 0 0 0 .1 .5 2)
SYMATTR InstName V1
SYMBOL voltage 656 224 R0
WINDOW 123 0 0 Left 0
WINDOW 39 24 132 Left 0
SYMATTR SpiceLine Rser=0
SYMATTR InstName V2
SYMATTR Value 5
SYMBOL res -272 320 R0
SYMATTR InstName R1
SYMATTR Value 1000
SYMBOL res 416 176 R0
SYMATTR InstName R2
SYMATTR Value 1e6
SYMBOL cap 416 352 R0
SYMATTR InstName C1
SYMATTR Value 1e-6
SYMBOL pnp 272 480 M180
SYMATTR InstName Q1
SYMATTR Value 2N4403
SYMBOL res 160 416 R90
WINDOW 0 63 58 VBottom 0
WINDOW 3 67 59 VTop 0
SYMATTR InstName R3
SYMATTR Value 1000
TEXT -354 616 Left 0 !.tran 0 5s 0 .001s

--
John Fields
Professional Circuit Designer

John Fields · Dec 27, 2005

On Tue, 27 Dec 2005 07:35:17 -0600, John Fields
<jfields@austininstruments.com> wrote:

On Mon, 26 Dec 2005 18:15:48 -0600, John Fields
jfields@austininstruments.com> wrote:

Aarghhh!!!

There's a gross error in the circuit which I'll fix tomorrow...

Hint: Vbe of the NPN won't allow the MOSFET to turn on, duh...

---
KISS works best...

+V
|
+-------+
| |8
[Rt] +---+---+
| 2|_ Vcc |3
IN>---+----------|--O|T OUT|--> OUT
| | 6| |
| +---|TH |
[1000] | 7|_ 7555|
| +-----+--O|D |
| | +| | GND |
| E [Ct] +---+---+
+--B PNP | |1
C | |
| | |
+-----+-------+
|
GND

Unlike the vanilla 7555 which starts timing out when pin 2 goes low,
with the retrigger it starts timing out when pin 2 goes high. That
means that pin 3 will be high for the timeout period (1.4RC) plus as
long as pin 2 stays low:

A caveat: most jellybean PNP transistors have an emitter-base
breakdown voltage of around 7V, and are rated at around 5V absolute
max, so if you run the timer with a Vcc of >= 5V it would be prudent
to put a diode in series with the emitter:

+V
|
+-------+
| |8
[Rt] +---+---+
| 2|_ Vcc |3
IN>---+-------------------|--O|T OUT|--> OUT
| | 6| |
| +---|TH |
[1000] | 7|_ 7555|
| +--[<1n4148]---+--O|D |
| | +| | GND |
| E [Ct] +---+---+
+--B PNP | |1
C | |
| | |
+--------------+-------+
|
GND

--
John Fields
Professional Circuit Designer

Specific 555 circuit

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