Can anyone tell me with this isn't working?

ericgibbs

Joined Jan 29, 2010
9,531
hi Halsey,
Welcome to AAC.
If pin #7 of the 3905 is holding up/down of the 555 pin 2 trig it will not work.
Use a connection cap 10n from pin 7 to pin 2 and a 10k pull up on 555 pin2.
E

Correction pin#7 of the 3905.
 
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Thread Starter

Halsey

Joined Mar 4, 2018
30
hi Halsey,
Welcome to AAC.
If pin #7 of the 3905 is holding up/down of the 555 pin 2 trig it will not work.
Use a connection cap 10n from pin 7 to pin 2 and a 10k pull up on 555 pin2.
E

Correction pin#7 of the 3905.
Is there a diagram I could see?
 

ericgibbs

Joined Jan 29, 2010
9,531
hi,
Look at this, I am assuming that 3905 Timer output goes low at the end of the time period, if not ,you need a transistor inverter.
E
EDIT: Added the transistor version
 

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Thread Starter

Halsey

Joined Mar 4, 2018
30
hi Halsey,
Welcome to AAC.
If pin #7 of the 3905 is holding up/down of the 555 pin 2 trig it will not work.
Use a connection cap 10n from pin 7 to pin 2 and a 10k pull up on 555 pin2.
E

Correction pin#7 of the 3905.
hi,
Look at this, I am assuming that 3905 Timer output goes low at the end of the time period, if not ,you need a transistor inverter.
E
Thank you very much for your assistance.
 

LesJones

Joined Jan 8, 2017
2,493
Hi Eric,
I think you may have misinterpreted the TS's requirement. My understanding is that he wants the 555 to give a burst of pulses with the duration of the burst of pulses controlled by the 3905. His schematic and yours seem to be triggering the 555 in monostabe mode giving a single pulse when the 3905 times out . Pin 2 will need to be connected to pin 6 for astable mode. I would think using the output of the 3905 to gontrol the 555 via it's reset pin would probably do what he wants.

Les.
 
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ericgibbs

Joined Jan 29, 2010
9,531
hi Les,
You could be right, I assumed he was trying to trigger the 555, by using pin 2 Trigger.
I wonder where he got that circuit.?
E
 

AnalogKid

Joined Aug 1, 2013
8,298
a 7555 CMOS in astable mode
First, your 555 circuit is not an astable oscillator. The 7555 datasheet has two different circuits for this. I think you should correct your 555 circuit first. Only then will any kind of control circuit work for you.

ak
 

Alec_t

Joined Sep 17, 2013
10,900
10Meg and a 360uF electrolytic are asking rather a lot of an accurate timing circuit, because (a) the current through the resistor is likely to be of the same order as the leakage current of the capacitor and (b) circuit board contaminants will be in parallel with the resistor.
 

Thread Starter

Halsey

Joined Mar 4, 2018
30
Sorry, the 3905 is supposed to trigger the 7555 CMOS in astable mode for a duration of one hour. The load is going to be earphones and LED lights.
 

Thread Starter

Halsey

Joined Mar 4, 2018
30
10Meg and a 360uF electrolytic are asking rather a lot of an accurate timing circuit, because (a) the current through the resistor is likely to be of the same order as the leakage current of the capacitor and (b) circuit board contaminants will be in parallel with the resistor.
The 3905 Timer was chosen for its precision which it has allegedly for up to several hours,as well as being immune to fluctuations in the power supply which will have no effect on its acuracy. It can't really operate well in astable mode though.
 

MrChips

Joined Oct 2, 2009
20,185
Sorry, the 3905 is supposed to trigger the 7555 CMOS in astable mode for a duration of one hour. The load is going to be earphones and LED lights.
It is still not clear what you are trying to do.

Do you want the output to last for a duration of 1-hour?
Or do you want the output to be triggered at intervals of 1-hour?

How long does the output last?
What does the output do? Flash or oscillate for how long at what frequency?
 

AnalogKid

Joined Aug 1, 2013
8,298
Once you have corrected your 555 astable circuit to be the same as one of the circuits on the datasheet, connect 3905 pin 7 to 555 pin 4. Do NOT connect 555 pin 4 to pin 8 or Vcc. When power is applied to the circuit, the 555 will oscillate for 1 hour and then stop.

Note: You must add a 1K resistor from 3905 pin 7 to GND.

ak
 
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Thread Starter

Halsey

Joined Mar 4, 2018
30
It is still not clear what you are trying to do.

Do you want the output to last for a duration of 1-hour?
Or do you want the output to be triggered at intervals of 1-hour?

How long does the output last?
What does the output do? Flash or oscillate for how long at what frequency?
The 3905 is to trigger the 7555 CMOS at 40Hz astable for 1 hour.
 

Thread Starter

Halsey

Joined Mar 4, 2018
30
The 20K Ohm trimmer is 20 turn to adjust it to 40Hz to a value of I think 13.5K Ohm to get an even sqaure wave.
 
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