555 Circuit somehow grounded

Thread Starter

SamR

Joined Mar 19, 2019
5,097
Old Forrest Mims circuit... Note that the red power lead is not connected in the photo, it is hanging loose/disconnected. When power applied the capacitor starts charging. And therein lies my problem. Connected MM to pin6. When I pull the chip C1 does charge, V ramps up like it should. With chip inserted 0V. Why is the chip grounding the circuit? Yes, I replaced the chip with the same results. I tried pulling the xstor w/ same results. SPDT switch not clear in photo but it does switch BC open/closed and B grounded. I am a bit confused by PNP...

screenshot.jpg event-failure-alarm.png IMG_0368.JPG
 

Alec_t

Joined Sep 17, 2013
14,411
It's a poor circuit design. If the wiper of the pot is set very close to the end connected to pin 6 and SW1 is closed, there is a chance that the pot's resistive track and/or Q1 will be destroyed.
Why is the chip grounding the circuit?
Pin 7 will hold the cap voltage down until pin 2 is taken low by closing SW1.
The +ve power supply to the IC is missing from the breadboard.
 
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Thread Starter

SamR

Joined Mar 19, 2019
5,097
The +ve power supply to the IC is missing from the breadboard.
Don't understand, sorry. +Ve? on what pin? And the buzzer is missing since it was annoying while testing. Ahh... Yes the positive lead is hanging loose in the photo. Closing and opening the switch has no effect on pin6/7 voltage.
 
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sghioto

Joined Dec 31, 2017
5,430
Remove the jumper from pin6 and pin7. The 2N3906 transistor is there to discharge the cap when the switch is closed.
Also as Alec_t mentioned there is a flaw. A 1K resistor should be added in series with the 1M pot to pin6 to prevent a short circuit when the pot is at minimum resistance.
SG
 
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Thread Starter

SamR

Joined Mar 19, 2019
5,097
OK 1K added from +rail to pot. That's a new trick for me, but I do understand it and will use it going forward. I do start pots at midrange and tweak when powered up, but I like the added protection.

Removing pin6 to 7 jumper I do not understand and had no effect.
 

sghioto

Joined Dec 31, 2017
5,430
This is how the circuit should be wired. I replaced the buzzer with a LED.
When first powered up the LED is OFF. The LED turns ON when the timing cycle reaches 2/3 Vcc unless the switch is closed first.
SG
EEE 555 event failure alarm.PNG
 

sghioto

Joined Dec 31, 2017
5,430
When power is applied LED will be on...
In the original circuit YES. Not if pin7 is left unconnected as in post #8.
EDIT: I'm sorry I meant with the switch closed.
The circuit is described as an "event failure alarm" the event is the closed switch.
SG
 
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Thread Starter

SamR

Joined Mar 19, 2019
5,097
This is how the circuit should be wired.
View attachment 173182
That's what I changed to except I put the 1K between the V rail and pot, Now added the LED for visual confirmation and moved the 1K to other side of pot. Cap still not charging and output pin3 stays ON. I'm going to build a quick 555 test ckt and check the chips. I think I bought these timers back in the 70's when I was building an Iambic memory keyer for my ham rig. I've trashed some over the years, but still have some good ones.
 

eetech00

Joined Jun 8, 2013
4,056
That lets the cap charge, but pin3 still on. I did add a 1u cap from GND rail to pin4 for start suppression.
If pin 6 is connected to pin 7, it will only let the cap charge if the reset pin is pulled high and the switch is open.
When the switch is closed (assuming its connected to ground), it will discharge the cap and set the output low. The reset pin overrides the trigger pin.

The internal 555 output circuit is a push pull configuration. So how an LED connected to the output lights depends on whether its referenced to VCC or ground.

eT
 
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