Hi All,
I have been trying to design a simple DC circuit that will give a half a second positive pulse 30-70 seconds after a momentary button is pressed.
The design context is a small free flight (ie non radio control) model aeroplane. The model climbs for 8 seconds on a timed motor run and then glides, but to prevent it flying away when it finds lift, it needs a 'De-thermaliser' (a.k.a. a DT.) This is some means of flipping up the tailplane to bring the model back down after a set time. There are mechanical options but I've been trying to create a electronic circuit to reduce the weight of the model.
The motor runs on a 200mAh one cell 3.7v Lipo, so I have sufficient power to run a 'spin off' micro coreless motor or to pulse a nichrome rubber band burner. The motor timer (my own design) is started by pressing a momentary switch which also provides a positive trigger for the DT circuit (the diagram also shows an independent start switch.) I am etching my own circuit boards and working with SMD parts down to about 0805 and SOT-23. Every fraction of a gram saved is important so a tight lay out and the smallest possible component count is helpful.
I've come up with the following sequential timer using two MIC1555 (SOT-23-5) chips https://uk.rs-online.com/web/p/timer-circuits/9101733. This is based on the standard double 555 circuit here: http://www.bowdenshobbycircuits.info/page9.htm#555-T.gif
This is working well but there is an issue: it triggers the burner pulse when attaching the battery ie on power up. This is just an immediate second stage pulse - ie no delay. This is obviously not ideal with a nichrome burner! Please can anyone suggest a way of preventing this or point to something I may have missed? The MIC1555 does not have a RESET pin like the standard 555 so I'm not quite sure what is happening.
If I can solve this power up triggering I have a functional circuit. But I'd also be interested to hear if there are simpler ways to achieve the same result - especially with fewer components.
Thanks in advance!
Jon
I have been trying to design a simple DC circuit that will give a half a second positive pulse 30-70 seconds after a momentary button is pressed.
The design context is a small free flight (ie non radio control) model aeroplane. The model climbs for 8 seconds on a timed motor run and then glides, but to prevent it flying away when it finds lift, it needs a 'De-thermaliser' (a.k.a. a DT.) This is some means of flipping up the tailplane to bring the model back down after a set time. There are mechanical options but I've been trying to create a electronic circuit to reduce the weight of the model.
The motor runs on a 200mAh one cell 3.7v Lipo, so I have sufficient power to run a 'spin off' micro coreless motor or to pulse a nichrome rubber band burner. The motor timer (my own design) is started by pressing a momentary switch which also provides a positive trigger for the DT circuit (the diagram also shows an independent start switch.) I am etching my own circuit boards and working with SMD parts down to about 0805 and SOT-23. Every fraction of a gram saved is important so a tight lay out and the smallest possible component count is helpful.
I've come up with the following sequential timer using two MIC1555 (SOT-23-5) chips https://uk.rs-online.com/web/p/timer-circuits/9101733. This is based on the standard double 555 circuit here: http://www.bowdenshobbycircuits.info/page9.htm#555-T.gif
This is working well but there is an issue: it triggers the burner pulse when attaching the battery ie on power up. This is just an immediate second stage pulse - ie no delay. This is obviously not ideal with a nichrome burner! Please can anyone suggest a way of preventing this or point to something I may have missed? The MIC1555 does not have a RESET pin like the standard 555 so I'm not quite sure what is happening.
If I can solve this power up triggering I have a functional circuit. But I'd also be interested to hear if there are simpler ways to achieve the same result - especially with fewer components.
Thanks in advance!
Jon
Attachments
-
86.1 KB Views: 28