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trying to trigger a TLC555

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Iowa Tom

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Hello,

I am trying to trigger a 555 timer chip (hooked up as a one shot) using the output (pin 12) from a decade counter 4017B to the #2 trigger pin of the 555. When I hook the counter up to the 555 directly (straight wire) the 555 seems to be triggered all the time.

Info for the 4017B can be obtained here: **broken link removed**

I am using a 9V supply.

I would greatly appreciate any help in getting this to work, one way or another! I want to use the 555 to power up a relay for about a second.

Thanks!

Tom
 
Iowa Tom said:
... a decade counter 4015B ...
I expect you meant 4017B, since you linked to that datasheet (and a 4015B is a dual shift register) ...
Anyway, it sounds like you need capacitive coupling. Place a capacitor (0.1uF or so) between your 4017's output and the 555's trigger pin. Also, connect a pull-up resistor (4.7Kohm) from the 555's trigger pin to V+ (Pin 8). Whenever the 4017's output goes low, it will produce a short, low-going pulse on the trigger pin of the 555, instead of holding the trigger pin low, which is what is happening. Also, by my datasheet, the 4017's "carry" output (pin 12) goes low at "5", so that's when your 555 will trigger, if that's what you want ...
Hope this helps, JB
 
Thanks for the help! I fixed the typo.

I think I have a bad chip too. Tomorrow I will get a new one and try again. I'll let you know how it comes out.
 
JB,

You said, "Also, by my datasheet, the 4017's "carry" output (pin 12) goes low at "5", so that's when your 555 will trigger, if that's what you want."

Which pin will go low at 10? I noticed that that pin went low at 5 but I convinced myself that I was misinterpreting what "divide by 10" meant.

Thanks,

Tom
 
Hi Tom,
On a 4017, none of the pins goes low only at "10" (actually 9, it counts 0 thru 9). You could do this a couple of different ways, though. You could connect an inverter (4049 or 4069) to the output that you want to trigger your one-shot from, then into your coupling capacitor to your 555's trigger. This is probably easiest. If you don't need the extra 5 sections of the inverter chip for something, connect the inputs to V+.
Another (kinda neat) way which could work is to basically make a "diode-OR" gate. Get 9 diodes (cheap ones, 1N4148, etc.). Connect the anode of each diode to the outputs 0-8, then connect all the cathodes together. Connect a resistor (4.7K or so) to ground from all those cathodes. Connect the cathodes/resistor to the input of your coupling capacitor. This way, when any of the outputs 0-8 are high, the output of this circuit will be high. When the counter rolls over to output 9, the output will be pulled low by the resistor. This is a "passive" circuit, no power req'd. (see drawing below).
JB
 

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