Author Topic: Trigger 555 from Active Low  (Read 866 times)

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Offline praveen_khmTopic starter

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Trigger 555 from Active Low
« on: November 07, 2013, 08:32:19 AM »
Hi,

I have a IR sensor (5V module) where output is generally high and drops low when it senses infrared signal (just like any other IR modules). Now, I plan to connect this to another (external) 555 module connected to a buzzer. This is where the problem starts. Using 555 in Astable mode, I can trigger it by connecting 5V to reset pin. Since the output is low from the sensor, I am not able to make out how to activate it.

The attached schematic shows the solution which I have found to be working (in simulation). Is there a simple way to achieve the same result? Also, is there a problem in this circuit?

Appreciate if somebody can please help.
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Offline waltr

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Re: Trigger 555 from Active Low
« Reply #1 on: November 07, 2013, 01:16:57 PM »
The transistor invert the signal level so it does do what you want. If the signal and 555 are both at logic Voltages then you could use one of logic inverter IC (74xx04 type). But if these are at high Voltages then the transistor is a good choice.

Offline jwatte

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Re: Trigger 555 from Active Low
« Reply #2 on: November 07, 2013, 01:53:38 PM »
I agree with Waltr: You can use a simple N-channel CMOS transistor like a 1N2222 or BS170, with a drain pull-up resistor, to invert the active-low input signal.

Seems like CircuitLab is no longer free, so I can't create a simple circuit to show, but it really is just input into gate, source to ground, drain to 2-10 kOhm pull-up resistor to VCC, and the output is the drain/resistor junction.

Offline praveen_khmTopic starter

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Re: Trigger 555 from Active Low
« Reply #3 on: November 08, 2013, 01:28:30 PM »
Thanks for the response waltr and jwatte.

@jwatte,

Are you suggesting the design I have attached, or is there something I am missing in that? (except for using 2222 instead of 3904)
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Offline jwatte

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Re: Trigger 555 from Active Low
« Reply #4 on: November 08, 2013, 03:16:15 PM »
Yes; use a 2222 and swap R5 and R6 -- with MOSFETs you want low resistance into the gate to quickly charge it, because it won't draw any current once it's turned on anyway. Meanwhile, 10k pull-up should be enough to turn on or off the 555, and a weaker pull-up wastes less current while the transistor is on.

Offline johnwarfin

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Re: Trigger 555 from Active Low
« Reply #5 on: November 09, 2013, 06:13:46 AM »
using a cheapo 2n7000 transistor instead of crummy bipolar allows replacing the 555 completely. simple rc one-shot.

Offline jwatte

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Re: Trigger 555 from Active Low
« Reply #6 on: November 09, 2013, 06:12:43 PM »
And John is right, I meant a 2n7000, not a 2n2222. I keep confusing those (because I generally just use BS170 :-)

Offline johnwarfin

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Re: Trigger 555 from Active Low
« Reply #7 on: November 09, 2013, 10:22:35 PM »
actually my point was to replace the far more complicated 555 circuit with a single 2n7000 with cap and resistor to form a one shot. if you insist on using a 555 with an inverter just about any transistor, bipolar or fet, would work fine. or maybe an off the shelf 555 based device might perform the function for 2 bucks with little modification:

http://www.ebay.com/itm/DC-12V-Delay-Timer-Switch-Module-Adjustable-0-to-10-Seconds-NE555-Oscillator-/130984339334?pt=LH_DefaultDomain_0&hash=item1e7f466786

Offline jwatte

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Re: Trigger 555 from Active Low
« Reply #8 on: November 10, 2013, 12:41:58 PM »
I presume the reason to use a 555 in astable mode to drive a buzzer, is to actually generate the buzzing tone. A single transistor won't really do that for you.
If the buzzer would actually buzz on its own with voltage applied, then clearly all you need is the buffer/inverter transistor.

Offline johnwarfin

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Re: Trigger 555 from Active Low
« Reply #9 on: November 12, 2013, 12:40:26 PM »
op and his diagram indicated basic astable and requested "simpler solution". hence my suggestion to reduce component count 5:1 by using fet instead of ic.

Offline jwatte

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Re: Trigger 555 from Active Low
« Reply #10 on: November 12, 2013, 06:28:18 PM »
Maybe I can learn something here!
How is a single transistor going to replace an astable (oscillating) circuit?


Offline johnwarfin

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Re: Trigger 555 from Active Low
« Reply #11 on: November 12, 2013, 06:42:40 PM »
ive misused the term "astable". op caused some confusion when referring to "triggering" it which is something done for a MONOSTABLE (one-shot). you dont trigger an astable which is more likely what he was referring to. the astable was the second 555 he mentioned but  had nothing to do with his problem or this discussion.

anyway in addition to implementing one-shot with rc and transistor its just as easy to do an oscillator (astable) with one. im sure you recall that now but if not let me know and ill put up a circuit.

the real shame is this could be done much cheaper and even fewer components (1) with pic or avr which do a great job of simulating dual 555. a lot more stable and virtually infinite delay times compared to 555. unfortunately most hackers suffer from "micro-phobia", fear of programmable chips.

Offline jwatte

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Re: Trigger 555 from Active Low
« Reply #12 on: November 13, 2013, 10:42:42 AM »
I see. I took "astable" to mean that the 555 was generating the actual tone of the buzzer.

I agree that a single 8-pin DIP can do what you need here (as long as drive current requirements are reasonable.)
Personally, I prefer the ATTiny85, but there are many good micros (like the PIC, or some MSP430s, or whatnot.)

The single transistor oscillators I know of require an inductor or a crystal, which also doesn't reduce part count or size :-)

Offline johnwarfin

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Re: Trigger 555 from Active Low
« Reply #13 on: November 13, 2013, 03:18:33 PM »
xtal? nope we are talking speaker here not rf. its easy to do a one transistor oscillator using a cap and resistor with enough drive for a buzzer as seen in the attachment below. or more conventionally with couple caps using pi network.

Offline jwatte

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Re: Trigger 555 from Active Low
« Reply #14 on: November 14, 2013, 11:27:57 AM »
Using the speaker as inductor -- got it! I agree with the assessment that the loudness will be low, though :-)

Offline johnwarfin

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Re: Trigger 555 from Active Low
« Reply #15 on: November 14, 2013, 06:08:33 PM »
actually its junction avalanche effect not inductance behind that. it still oscillates with only a resistor in the circuit. more conventional single transistor circuits would depend on phase shift instead but do require one or two additional passives. to be honest i would rather use a 555 or better still a couple logic gates for stabilty and interfacing benefits.

we definitely agree a cheap mcu is best solution. m85 also happens to be my favorite avr too. although for absolute lowest cost a t13 or even one of the 6 pin t8, t9, t10, etc. interestingly one of my clients had me code an m8 to emulate eleven 555 timers in one package. i remember when having at least one 555 in your circuit was a requirement for publication in nuts and volts magazine. The Chip That Wouldnt Die! coming to a theater near you. lol.

 


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