Small Problem with Backlight Timer

[Leslie Milton]

Hi all
I have seen this circuit which is based on the 555 monostable its purpose is to add a back light to a radio aircraft transmitter screen, I have built the circuit on a breadboard but it does not work so I used Proteus 8 professional to run a simulation of the circuit and although it worked it did not operate as expected, the problem was that when the circuit was started the back-light (an LED) did not light up until the trigger (marked buzzer) contact was made then the LED lit for the set time and continued to light when the contact was made.
In an attempt to get the back-light to operate when first powered on I tried changing some of the components, when I used an animated capacitor which Proteus has in its components to see what was happening when the circuit powered on the circuit worked as it should. I have tried many different types of caps in the simulation but none work as the animated one, unfortunately this cap has no identification it is listed as animated so I have no idea what to use in its place I don't want to build the circuit until I know the required part and it works, I have attached a copy of the original circuit and a screen capture of the working circuit in Proteus any help with this would be gratefully received.
Thanks Les
**broken link removed**

 

[D.A.(Tony)Stewart]

Q1 is not needed, if you use a momentary switch to ground on the Trigger input..

To make the circuit trigger on power up, connect the (+) of cap to V+ instead of (-) to ground with other end on Threshold. (unlike as shown below). Then put a cap on Control to ground to force a SET on power up.

When there is no RESET , Set requires TRIGGER higher than CONTROL. Check Chip schematic

Give that a try. ( I personally have never used 555 timers in any of my designs :lol: )

 

[Audioguru]

Is your switch still turned on when you want the timing to end? the datasheet for a 555 says that the trigger input to ground must go high BEFORE the timing will end. The datasheet from Philips shows how to capacitor-couple the switch so it makes a short pulse to trigger the timing so the timing can end even if the switch is still turned on:

 

[D.A.(Tony)Stewart]

He wants to force Set on Power Up with Cap initially discharged but tied to Vcc.

rather than force Reset while switch is depressed. Did I get that Right?

 

[Leslie Milton]

Hi Thanks for the response to my problem I have tried to follow the advice given but I am having trouble fully understanding what I am reading I am a basic novice to electronics I have only taken an active interest since retiring, I am able to follow circuit diagrams and solder well but really need special needs level of help.
This circuit works in Proteus with the exception of the back light not coming on at power up, unless I use the animated cap then it lights at power up. The switch used on the R C transmitter that triggers the back light is a roller type to scroll through the settings which is then pushed to select, I have used a momentary button in simulation which I assume is similar in operation.
Thank you all for your Patients.
Les