Hi guys

Hope my drawing makes sense, I would like to mount a timer on a rocket, and the timer will only be activated after it pass the sensor.

I am hoping the timer can only be activated by the sensor, and the top speed of the rocket is 100m/s, it's about 50cm long.

Hopefully there is a chip can do something like this??

Thanks guys!

 

You could use a photocell, micro-switch, opto coupler and led, infra red beam breaker.

 

You could use a photocell, micro-switch, opto coupler and led, infra red beam breaker.

Hi Dodgydave, could you give a bit more info on how you could do this?

 

I'd probably go with an IR led/phototransistor which would be placed at the opening of the chamber . I'd connect it to a comparator or maybe even straight to an MCU input pin which would start fire an interrrupt starting the timer when the pin goes low or high .

 

What is your timer going to be ?
Are you using an MCU?
I'd probably go with an IR led/phototransistor which would be placed at the opening of the chamber . I'd connect it to a comparator or another transistor.

Yes, I am planing to use a MCU, but I still don't understand how to active the timer, which is mounted on the rocket (not the chamber) by an IR LED etc..

 

Tried it on the breadboard and it works fine .

With the sensor active (nothing blocking the path between the IR LED and the IR phototransistor) there is about 4,96 volts on the output and when obstructed there is about 0.030 volts on the output .
The circuit uses a general purpose NPN transistor.
Your R1 value might vary with your phototransistor values

So you could probably attach that straight to an MCU input pin.

 

oh you want to activate it on the rocket...
Hmm going to have to think about that one

 

Well why don't you just simply attatch a veeery thin wire loosely with some tape to a contact on the rocket with +5volts and when the rocket starts , it will rip the wire off which will pull your input pin on the MCU on the rocket low and start the timer.
pretty easy way to do it

 

Or another way you could probably do it is mounting an LDR (light dependant resistor) on the rocket .
Inside the chamber it would be dark so if you put an LDR in the circuit that I drew above then you would get an output of 5V and when the rocket comes out of the chamber and sees light then the output would be almost 0V.
You can programm the timer to start when the rocket sees light

 

magnetic reed switch.

attach the magnet to the rocket so that the switch is closed/or open(depends on the circuit you end up using) and attach the magnet to a strong piece of nylon fishing line. When the rocket takes off the fishing line wire will dislodge the magnet from the rocket and operate the switch.

reed switches are very small and light so this would be ideal for a such a small lightweight platform with a simple timing circuit. I would use the switch to supply power to the circuit when the magnet is removed. two small 3 volt button cell batteries would be a good choice for a power supply.

 

magnetic reed switch.

attach the magnet to the rocket so that the switch is closed/or open(depends on the circuit you end up using) and attach the magnet to a strong piece of nylon fishing line. When the rocket takes off the fishing line wire will dislodge the magnet from the rocket and operate the switch.

reed switches are very small and light so this would be ideal for a such a small lightweight platform with a simple timing circuit. I would use the switch to supply power to the circuit when the magnet is removed. two small 3 volt button cell batteries would be a good choice for a power supply.

Well why don't you just simply attatch a veeery thin wire loosely with some tape to a contact on the rocket with +5volts and when the rocket starts , it will rip the wire off which will pull your input pin on the MCU on the rocket low and start the timer.
pretty easy way to do it

These two solution is easy and simple, but I am concerned that they are not going to be very reliable. But you guys ideas gave me a good direction, thanks.

Or another way you could probably do it is mounting an LDR (light dependant resistor) on the rocket .
Inside the chamber it would be dark so if you put an LDR in the circuit that I drew above then you would get an output of 5V and when the rocket comes out of the chamber and sees light then the output would be almost 0V.
You can programm the timer to start when the rocket sees light

I actually like this idea, but I think the timer will be activated before loading the rocket as well. I think I will need to find a reliable way how to not activate the timer before loading.

 

I think I will need to find a reliable way how to not activate the timer before loading.

With the reed relay, you can have the timer reset whenever the reed is activated by the magnet.

 

With the reed relay, you can have the timer reset whenever the reed is activated by the magnet.

I should have mention the timer is going to be less than 5s, if someone just forget to reset it within 5s, horrible thing is going to happen , I don't want that.

 

I should have mention the timer is going to be less than 5s, if someone just forget to reset it within 5s, horrible thing is going to happen , I don't want that.

This 'horrible thing' wouldn't be associated with a loud noise, would it ?

 

This 'horrible thing' wouldn't be associated with a loud noise, would it ?

let's say I am planing to implement a second switch after the timer starts, it will ONLY activate 4-5 meter away from the launcher.

Now you can have a wild guess

 

A nice secondary safety would be to use the mentioned long string/wire to pull out a non-conductive material blocking the power contacts in the rocket only when it reaches maybe even 10 meters or more.. Is that what you were thinking ?

But, it would be prudent to be prepared for ANY malfunction.

 

A nice secondary safety would be to use the mentioned long string/wire to pull out a non-conductive material blocking the power contacts in the rocket only when it reaches maybe even 10 meters or more.. Is that what you were thinking ?

But, it would be prudent to be prepared for ANY malfunction.

I haven't thought of it that far ahead yet, I just solving the problem one at a time the the moment. I do know I will need some sort of safety thing in place.(maybe a mechanical solution, I will see what my mechanical guy can come up with)

 

This 'horrible thing' wouldn't be associated with a loud noise, would it ?

Is there something I can buy on the market, do you know? If there is, it will be silly for me to make one.