Simple electronic timer

[Lil_Guppy]

Since this is my first thread, I have placed it in the Water Cooler in case I breach any of the forum rules by accident. Here goes...

The technical info
Recently I began research on the net for a simple timer circuit that could be used to initiate a model rocket ignitor. I wanted something that had some indication of the time remaining, and could be modified to set off strings of ignitors at different times. I did a search in Google and I found an article written on here by Jumala with his (or her) timer diagram (this is actually how I found this forum :)). Unfortunately I couldn't find one of the parts where I am, so I looked further into the net. I found so many articles about the good ol' 555 timer, but this isnt what I really wanted, for several reasons, one being I wanted a set time to elapse *before* the IC's output went high. For those who dont know, when the 555 timer is activated, the output goes high for the time period, then goes back to low. I then came across the 4017 CMOS chip. Basically the 4017 is an IC that has 10 outputs switched by a clock pulse. What that means is that each output (1 - 10) is selected by a pulse on the clock input. So the first pulse would make output 1 go high, while all other outputs remain low. The next clock pulse turns output 2 on, and all others off etc etc. So... (apologies if this is hard to understand) I then set up the 555 in an astable mode and fed it into the clock trigger input of the 4017. Connected to the first 9 outputs are 9 LED's (one on each output) to give a countdown indication (In my prototype i had 3 green, 3 yellow and 3 red LED's). The 10th output was connected to the 4017 clock disable pin and via a transistor to a relay. The 4017's clock disable pin just means that no more clock pulses are counted, essentially freezing the count on 10, and freezing the relay on. Of course to the relay, the ignitor is connected. The timing period is defined by R1, R2 and C1. If anyone is interested in this circuit, please let me know and I will post more information on adjusting the timing. And I think thats about all the tech stuff at the moment.

Other Ideas
I said in the section above that I wanted the circuit to be adaptable and able to be used to set off strings of ignitors (or detonators etc etc) one after each other. By using this circuit, you can set up to 10 events by removing the indicator LED's and replacing them with the transistor/relay setup used on output 10. Just remember not to connect the first 9 outputs to the clock disable pin, otherwise the count will never get to 10.

At the moment, I dont have a web server that allows linking image files, so to find the circuit diagram, please click here (http://www25.brinkster.com/darkside01/layout1.html)

I hope that this info is of use to someone, and I hope that it is an acceptable thread.

[chemwarrior]

It is quite useful. Ive been working on a simple electric ignitor and this info is quite useful. It will benefit the members on here who dont have much knowledge of electronics, well, it may confuse them a bit because of the technical jargon, but Im sure they can find out what things mean on their own.

Also, you dont need to ask if anyone wants to see the circuit. Just up it somewhere and post a link.

On another note, this would have been perfect for the Tools, Techniques, and Plans section. Despite it not being advised to post a new thread as a first post, in this instance it would have been alowed I think, due to the fact that it was quite informative.

And please, post any more info you have on this circuit. It seems quite nice:)

Tuatara, do you have any info you could share with us? You seem to be the best qualified to determine if this is workable or not, as I only have a small knowledge of electronics....:(

[Lil_Guppy]

Well, basically the only other info needed is how to get different timing periods. Using the component values shown on the diagram will give about 1 minute until the relay is triggered. The following shows how to work out the component values for other time periods (I wont go into detail about why you do the steps at this time, as it may just confuse people even more :P)

Step 1
Figure out how many seconds delay you want/need.
e.g. - 120sec (2 minutes)

Step 2
Divide this value by 18
e.g. - 6.6666sec

Step 3
Divide the value from Step 2 by 0.07
e.g. - 95.238
This is the value of R2 in kohms.

Using this method, and keeping all other components the same, delays of as little as 12sec (R2 = 10kohms) or up to 21min (R2 = 1Mohms) can be obtained. You shouldnt go below 10kohm or above 10Mohm for R2, or else things get a little unstable. To get other values, C1 needs to be changed, but thats getting a little to confusing at the moment. Hope this helps.

[EDIT] I actually made a mistake when drawing the circuit diagram. It is fixed now. Just wanted to let everyone know in case they printed the old copy.

[zeocrash]

wow this is a very refreshing change. A new member who knows something.
Tutura is probably the best person to review your idea, but it looks very promising. it's nice to see a competent new member.

[Lil_Guppy]

I dont know about competent... :p

The prototype I did yesterday (yes I did actually build it) worked a treat. It didnt cost much to make the base unit (i.e. without switch or box). The time delay was consistant even though I was using resistors with a 5% tolerance. Damn electronics store ran out of the 1% ones :( but it didn't really matter because its only the prototype. Like I said above its pretty easy to adapt it for different purposes. For example, you could have a switch which selects between 2 timing periods, a switch to turn off the indicator LED's, or you could remove the LED's altogether and have 10 devices set off one after each other.

I do admit that I know little about a few of the other topics discused on this forum, but then again, thats why Im here, to learn. :)

[Bert]

We use electronic sequencers for pyrotechnic displays, the
typical timings are much shorter than those described
in Lil_guppy's post... Generally from 1/100ths of a second
to a couple of seconds. We use them for things like comet
chases, mine runs and other choreographed effects that
need better timing than can be provided by a human
finger on switches but don't call for a full scale computer
firing system. The are kind of pricey, typically $200.00
to $500.00 per box excluding the cabling and firing
slats or other terminals for the e-matches.


This plan looks interesting to me- For my uses I'd want to set
it up with a variable and lower range of timings most likely,
but that shouldn't be too tough.

Can you ballpark your cost of the basic circuit in US $'s?

Have you put any thought into output circuitry beyond the
relay switching the firing power? Commercial, field grade
equipment needs protection from over current, switch (relay
contact) welding, firing into dead shorts or excessive #'s of
parallel igniters and all the other things that can happen when
idiots use a firing system. Additionally, protection from
induced currents in the output circuit can be a benefit
in prolonging the user's life expectancy... Shunting
or other protection to keep the setup from firing due to
induced voltage, be it from radio or high currents in an adjacent
firing line.

Also, a built in continuity indicator is a very nice thing to have
in any firing system (I don't ask for much, do I?!)

Thanks for this high quality first post!

[Hubert]

Rusian timers :cool:

http://www.pirotex.by.ru/7.htm

I have some fotos of very easy timers, maby i past this photos in to the forum, when I have more time for this stuff. :)

[Tuatara]

I'd give it two thumbs up. A nice simple circuit, readily available (cheap) parts. I think the only thing I'd change is to insert a 1k resistor between the RST pin (15) and 0V and add a 'Reset' button (normally open contact) between RST and VDD. This is simply because 4017's do not have a guaranteed starting state, a reset button allows you to zero the system.

By the same token, a switch in line with the relay coil would be a useful safety feature, when powering up the circuit to avoid accidental firing

[Lil_Guppy]

Bert
Changing the timings to a lower setting is really easy, as all that needs to be replaced is C1, which is changed to a a lower value (10uF perhaps). Of course the steps to work out R2 I wrote above wont work though but thats not a problem. Ballpark figure for the basic circuit was around AU$12. The basic circuit offers no protection from the things you mentioned, but the relay could be easily replaced by a FET which removes the risk of contact welding. As for the continuity tester, that would be rather simple to implement.

Tuatara
I did think of adding the stuff you mentioned, but I had already bought the bits and came home :p

[Bert]

When I get a moment, I'll copy the output circuitry from
my manual firing system. Includes an e-match safe
continuity check, shunting, current limiting, etc. Shouldn't
raise the cost too much, I hope.

[Lil_Guppy]

That would be quite handy actually if you could get that info to me. I dont think that it should raise the cost much, although one would never know :p If I can get other ideas/info, perhaps I should create a little booklet for it, with all the information to make different timing periods, different variations on the circuit for different uses etc etc. Of course I would post it here for all to see :) Let me know what you think.

[Lil_Guppy]

Well... I just threw together a quick PDF file for the timer circuit (minus the images) and wanted to know if I should add/remove anything from it. The circuit diagram is still available from the link in the post at the top.

The PDF File (http://www25.brinkster.com/darkside01/downloads/timer_instructions.pdf) (Right click -> Save As)

Like I said, it doesn't have the circuit diagrams in it, as I am still in the process of drawing them.

[DBSP]

As my electronic skills are very limited I just have to ask you, what kind of bridgewires/ignitors can be used with this setup. The current needed for a model rocket ignitor is much less than for a nichrome ignitor. Could it handle nichrome igitors for instace?

Sorry for the stupid question but I can't really figure it out myself.

And secondly would it be able to connect this timer unit to my blasting box to fire a relay triggering a tyristor realeasing the electric impulse from my blasting box?

Specs on my blasting box below:

http://www.roguesci.org/theforum/showthread.php?s=&threadid=1927&highlight=blasting+box

The pictures are offline though.

[Tuatara]

The current rating of the relay contacts will determine what sort of ignitors you can use. Back to Ohm's Law:

I=V / R, where R is the resistance of your ignitor, V is the voltage you are applying to the ignitor, I is the current that will flow.

Make sure I is less than the DC current rating of the relay contacts and all should be well.

The answer to your second question is a very simple 'yes' :D

[Lil_Guppy]

Or replace the relay with a FET :P

[The ALL-MIGHTY]

I saw your post on that electronics form too, a timer would be a great, not as expensive as a radio detonator and much simpler!

What would really work good is if your could program it to any number you wanted and see it count down on a display. i saw some stuff like that in a google search. also anyone that knows elctronics should in some way download Circuitmaker, not becasue you can build stuff with it but because it has a really good model rocket igniter circuit and some other (maybe about 3) delay circuits, one of which has a 99-1 second time range.

[Tuatara]

Lil_Guppy, don't place too much faith in silicon! I've seen FETS protect fuses by blowing first:eek:
A relay contact is capable of withstanding a subtantial overload, the sort over overload that will make a FET detonate (and yes, I've seen that happen too - very nasty when you don't expect it)

This is not to say FETs can't be used, just that more design work is required ;)

[Lil_Guppy]

Tuatara
Indeed. I was just stating some options. I have had no troubles with using the relay as yet, even though it is only a small cheap 'hobby' relay (thats what its called in the catalog). I know all about explosive silicon as well... ;) Do you have any more suggestions that I can put into my PDF?

The ALL-MIGHTY
The reason I did this the way I did, was because it is cheap and easy to construct. Having a fully programmable timer with numbers for the countdown etc etc begin to take it into the not as cheep or easy to construct realm. As for circuit design software I use ExpressPCB. Its free, and pretty darn good. It comes with a schematic drawer and a PCB layout drawer which you can link back to the schematic. As for the software you mentioned coming with free scematics, I find I learn more if I have to search for the info, rather than having it spoon-fed ;):p

[Jumala]

Last days I looked for other timer chips like 4541 and I found a thread of the elektor magazine about a long period precision timer.
It works from seconds up to half a year. It´s made from one 4541 with crystal oscillator and two 4040 dividers.
The PDF file is 185 KB large.
If someone needs the file please send me a mail add.

To lil_guppy

The 4541 gives you a set time which elapsed before the output turns to "high".
In my plan the 4541 is the only inportant part. All others are exchangeable.
Which part isn´t available for you?

[Lil_Guppy]

Jumala
I did end up finding the bits for the timer you sent me. The reason why I decided on using the 4017/555 combo is that I can use the spare 9 outputs to drive indicator LED's to give an indication of how much time has elapsed, plus it can be easily modified to drive a sequence of up to 10 events, which could be useful for pyrotechnic displays.

[Lil_Guppy]

Just to let everyone know, I have created a much better PDF file for my timer. The download link is here (http://www25.brinkster.com/darkside01/downloads/timer_instructions.pdf) (Right Click -> Save As...) I hope this can be of some help to everyone :)

[grandyOse]

I started reading this thread because I love SIMPLE stuff. For your purpose (countdown timer) it looks like you have a winner. If I may, I'd like to add some ideas for a different SIMPLE counter.

For an accurate remote timer, such as a "timed-bomb" would require, or a device that turns on and off at at set time every day, or at set intervals, a cheap digital LCD clock can be used. You can have an output for each segment of the display. Choose the segment that turns on at the time, or rate of your disired output. Tracing down the connection to each segment can be a pain, but it can be done. with the battery removed, use the diode function of your digital VOM. Looking at the circuit board behind your display it should be obvious which is the common supply lead for each digit; they will be connected together, a trace connecting some pin or pins on each digit. The common may be either positive or negative. Hell, just poke around with your ohmeter leads and watch the display. A mirror can help with this. Soon you'll find out the polarity and pins for your desired segment or segments. You may want to run your outputs through some logic gates if you want some weird intervals that don't correspond to any single segment of the display. The display can be directly connected to logic gates, but to actually turn something on and off you will need an amplifier and possibly a reley.

I didn't want to steal your thread, but I didn't want to start a new one, either. Hope this is OK.

[Lil_Guppy]

grandyOse
Steal away :p If its a timer, then I dont mind. I must say that there are much simpler ways of doing a timer like the one you described. If all you are looking for is an on-off cycle that repeats itself, you could use a simple 555 timer circuit. If you want to sequence more than 2 things over and over again, you could even modify the circuit I presented so that when the count gets to 10, it resets. This is done by removing the link from between Pin 11 and Pin 13 on the 4017.

[T_Pyro]

I've used timers based on the 555 before, but these timers aren't exactly "expendable". Sometimes, simple electronic timers are required which have to be cheap, compact, as well as expendable. For such requirements, I'd designed an analogue timer circuit using minimal number of components.
The circuit diagram is available here (http://www.geocities.com/joydeep_b2002/Timer_Ckt.html) .

Components:
R1, R2, Rt: carbon composition resistors, values to be selected according to the timing required.
Ct, C1: Electrolytic capacitors. C1=1000microF minimum, 3000uF recomended, voltage rating same as Vcc. Ct value to be selected according to the timing required.
NPN silicon transistor
Vcc, battery source, 5V min.
1 DPST switch to charge C1 through, (use for the connections to C1) and 1 SPST switch to activate the timer.
Nichrome wire.

Equations:
Vb (base voltage)= (R2/(R1+R2))*Vcc
t (time delay)= Rt*Ct* Ln(Vcc/(Vb-0.6))

Working:
Initially, when both the switches are in the "on" position, both the capacitors are charged up to Vcc. Next, the DPST switch is released, and the connections to the nichrome wire made. The circuit starts as soon as the SPST switch is released. The capacitor Ct starts discharging through Rt, and when the voltage across Ct becomes 0.6V less than Vb, the transistor turns on, and the capacitor C1 discharges through the transistor and nichrome wire, igniting the explosive charge being used.

[grandyOse]

Have you tested this circuit? Could you label your switches or draw a dotted line to denote mechanical coupling on the DPST? It seems to me that to charge both caps, all three swithes must be on, but that would put Vb across the ignitor, bringing an abrupt end to the operator. Maybe I'm just not seeing the switches correctly.

Timing is going to be approximate, also. You could do a lot of bench testing to get the timing more precise. If you want something that's going to go off in "about 2 minutes" this is about the simplist type of circuit you could build.

I'm not trying to sell anything here, but that's where my timer is good. It has japanese quartz timing built in. But, heh, it's all good if it works. Whatever you have available and makes you smile, that's the best circuit.

[T_Pyro]

Yes, I've tested this circuit, and it works well enough for what it's worth. I had configured my timer to give a delay of 46 seconds, and it gave a time lapse of about 48 seconds on all the tests. The difference between the theoretical and experimental value could be because of many factors, but whatever the cause, the experimental value remained within 1 sec of 48 sec.

To make the switch connections clearer:
The left and right switches constitute the DPST switch, which is used to charge the ignition capacitor (C1).
The switch in the middle is the single SPST switch, and it initiates the timing circuit.
Sorry for the confusion.

To clarify the working of the circuit:
When all the three switches are on, both the capacitors charge up to Vcc. In this state, the nichrome wire should NOT be connected, else it will ignite in your hands.
When the DPST is off, the nichrome wire can be safely connected. As the SPST switch is still on, the capacitor Ct cannot discharge yet, as it is held across Vcc. When the SPST switch is switched off, Ct starts discharging, and the timing sequence has started.

grandyOse, like you stated, this circuit is not meant for precise timing, as it would have an error of +-1.5 sec (experimentally observed). Rather, this circuit is meant for occasions when you need to be far from the scene of the detonation, and would rather not return to the scene.

Bert, when you spoke of a "full scale computer firing system", exactly how sophisticated was the system? It's easy to make a computer-controlled firing sequence for far less, accurate to within 1 millisec, and capable of initiating 256 events, or when scaled up, 65536 events. If anyone is interested, I could post the plan, but I don't think it could be considered as a "simple timer", although the cost would be about 20 US$.

[grandyOse]

Yes it all makes sense now. I should have read your original post more carefully, for it states to hook up the ignitor AFTER charging the capacitors and turning off the charging switch. I'm suprised that the timing is so accurate, but glad it works for you. If I were going to build an RC timer, I would probably use a schmitt trigger whose input is the RC network. I'd also put the nichrome wire in the emmitter or collector current path of the output transistor. I'd use one switch to activate the timer, probably a DT which would keep a short across the cap in the off state (I'm pretty paranoid).