Transistor as switch -problem

Thread Starter

athulascc

Joined Aug 15, 2014
94
here is my led flasher circuit which is connected to timer.
I need this circuit to flash LED for 30sec and off.
I have used transistor to enable the flashing cct.
is that correctly done?
Capture.JPG
 

Thread Starter

athulascc

Joined Aug 15, 2014
94
I am going to switch whole flasher circuit. I am not sure it will works or not. I know there is not included base resistor, I can add it.
if there is any different method than this inform me.
 

ErnieM

Joined Apr 24, 2011
8,200
The concept should work. Practically you need a resistor in series with the base of Q1.

Without knowing what voltage this thing runs off of one could only guess at a value.

My guess is 500 to 1500 ohms.
 

WBahn

Joined Mar 31, 2012
26,398
I also agree. Switching power/ground has its uses, but you need to be pretty careful and deliberate about doing it because you become responsible for knowing how the I/O pins of a chip that is not powered properly will behave.
 

Thread Starter

athulascc

Joined Aug 15, 2014
94
ok
if reset pin is one circuit works fine
when it 0 flasher not working
I understood that,
but I changed circuit to this before see the comments
what do you think about this idea?Capture.JPG
 

Thread Starter

athulascc

Joined Aug 15, 2014
94
I have another question.
I do simulation in proteus 8.
But when I draw both 555 ics timer does not work .
it only on but do not off in 30sec.
but if I delete whole flasher part it works well.
also even when I do not give power to flasher it works,
what's wrong.
may I neglect simulation results?
 

WBahn

Joined Mar 31, 2012
26,398
Once again you are trying to switch the power to a part. Do you KNOW how a 555 is going to behave when it is part of a circuit but is not powered? Do you KNOW that the simulation model is going to accurately model that behavior?

If the answer to either of these is, "No," then don't do it!
 

Reloadron

Joined Jan 15, 2015
6,222
Also as drawn your second 555 which is configured as an astable multivibrator will have the following approximate characteristics:

Frequency: 48.1 Hz.
Cycle Time: 20.790 ms
Duty Cycle: 66.67%
Time High: 13.860 ms
Time Low: 6.930 ms

Those LEDs will be flickering very fast and likely just appear to be on to many. The 555 output will not be symmetrical with 2/3 of the cycle time one LED on and 1/3 of the cycle time the other LED on. Just so you are aware of that.

Ron
 

Reloadron

Joined Jan 15, 2015
6,222
OK, I am obviously missing something here with the reference to R5. The circuit in post #11 reflects using two 555 timers. The first, on the left is configured as a "One Shot" where when the start button is pressed and released the first 555 on the left will have its output on pin 3 swing high and it will remain high for the duration of its R*C timing network. The original poster wants to use this high out from the one shot to enable the second timer. The second timer is another 555 configured for an astable mode. The idea here seems to be to have a pair of LEDs on its output alternately blink while it is active or enabled.

The free running frequency of the second stage, the 555 running astable is determined by:
  • Increasing C1 will increase the cycle time (and hence, reduce the frequency).
  • Increasing R5 will increase Time High, but will leave Time Low unaffected.
  • Increasing R4 will increase Time High, increase time low and decrease the duty cycle (down to a minimum of 50%)
Now would that be correct? That is how I see things working. So what is this thing with R5? The only thing R5 does is increase the On Time of the output pulse. I do not see R5 as being able to enable or disable the second stage timer. What am I missing with this R5 part of the discussion?

Ron
 

ScottWang

Joined Aug 23, 2012
7,080
OK, I am obviously missing something here with the reference to R5. The circuit in post #11 reflects using two 555 timers. The first, on the left is configured as a "One Shot" where when the start button is pressed and released the first 555 on the left will have its output on pin 3 swing high and it will remain high for the duration of its R*C timing network. The original poster wants to use this high out from the one shot to enable the second timer. The second timer is another 555 configured for an astable mode. The idea here seems to be to have a pair of LEDs on its output alternately blink while it is active or enabled.

The free running frequency of the second stage, the 555 running astable is determined by:
  • Increasing C1 will increase the cycle time (and hence, reduce the frequency).
  • Increasing R5 will increase Time High, but will leave Time Low unaffected.
  • Increasing R4 will increase Time High, increase time low and decrease the duty cycle (down to a minimum of 50%)
Now would that be correct? That is how I see things working. So what is this thing with R5? The only thing R5 does is increase the On Time of the output pulse. I do not see R5 as being able to enable or disable the second stage timer. What am I missing with this R5 part of the discussion?

Ron
>I do not see R5 as being able to enable or disable the second stage timer.
If R5 connected to GND then the second 555 will stop and that is the first 555 want to do.
 

Reloadron

Joined Jan 15, 2015
6,222
>I do not see R5 as being able to enable or disable the second stage timer.
If R5 connected to GND then the second 555 will stop and that is the first 555 want to do.
Ah, so you are saying to ground R5 to disable the second timer. Place the junction of R5 and R4 at ground. Thank you as now I understand what you are suggesting.
You should move R5 not the reset pin.
Left me thinking move it to where? :) Now I understand what you are suggesting.

Thank You
Ron
 

ScottWang

Joined Aug 23, 2012
7,080
Ah, so you are saying to ground R5 to disable the second timer. Place the junction of R5 and R4 at ground. Thank you as now I understand what you are suggesting.

Left me thinking move it to where? :) Now I understand what you are suggesting.

Thank You
Ron
What I knew the methods to control a 555 astable to stop generate the pulse as :
1. Using a PNP bjt to control from Vcc(pin 8) with a low level in base.
2. Using an NPN bjt to control from Gnd(pin 1) with a high level in base.
3. Control the reset pin (pin 4) to Gnd.
4. Control R1 to Gnd. (see astable circuit)
 
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