Help with an electronic geocache project

Hello all, new here and looking for help and advice, of course...

I am thinking of a project for a geocache. This is a hobby where you hide a "cache" (usually a watertight plastic container or an ammo can) in the woods somewhere, post the coordinates online and other people go searching for it using a handheld GPS. The cache usually contains trinkets for trading and a logbook for recording the find. Search on geocaching if you're interested.

My project would be part of a multi-stage cache. Mounted inside this container would be a panel with a row of seven ON-ON toggle switches and some 7-segment LEDs. The finder would have to figure out the correct position for all the switches in order to display the coordinates of the next stage (there would be clues on the cache page to help with this). If the switches were set in any configuration except the correct one, the display would be random gibberish.

The first few coordinates would be given (printed on the panel), with the LEDs supplying the final three digits, like this:

N 38 34.XXX
W 076 12.XXX

with the LEDs sitting where the x's are.

I've poked around Digi-Key and come up with this for the display: https://www.electro-tech-online.com/custompdfs/2010/02/LDS-CA14RI.pdf

So I've got 42 segments to power, and 14 switches (7 two-position switches). So each switch position will light 3 segments, not necessarily on the same module. I'll wire it up to show the correct numbers when the switches are set properly, and randomly connect the other settings.

So on to the questions:

The datasheet for these displays shows a 4V forward and 5V reverse voltage. So I need a 5V power supply, right? Can I just stack three (or four?) 1.5V batteries together, or do I need a more complicated power supply?

Steady current is given as 30 mA. So is this just a current divider problem? 5V/0.03 = 167 ohms, so I should put 167 ohms in series for each segment?

Is there anything else that should be included on this simple circuit--capacitors, shunt resistors, other things I'm too ignorant to know about?

If you understand what I'm trying to do, is there anything that could improve the "coolness" factor of this, without making it too complicated (don't want people frustrated out in the middle of the woods on a multi-stage)?

Thanks in advance for any advice.

CB

CAPT Bligh said:

So on to the questions:
The datasheet for these displays shows a 4V forward and 5V reverse voltage. So I need a 5V power supply, right? Can I just stack three (or four?) 1.5V batteries together, or do I need a more complicated power supply?

Click to expand...

Yes you can just use 4 AA batteries in series. That will give you roughly 6V with fresh batteries. You could also use a 9V as long as the thing isn't on the whole time it is waiting to be found.

CAPT Bligh said:

Steady current is given as 30 mA. So is this just a current divider problem? 5V/0.03 = 167 ohms, so I should put 167 ohms in series for each segment?

Click to expand...

The 30mA is likely the maximum continuos forward current. I would use 25 mA. The segments will still be very visible at that current. To caluclate the correct resistor values use this equation:

Vs= Supply voltage (9V or 6V w/ 4 fresh AAs)
R = Resistor value
I = Current through each LED (25 mA or .025 A)
Vf = Forward voltage of each segment (4V for all segments except 2V for decimal point)

V = IR

R = V/I

R = (Vs - Vf)/I

CAPT Bligh said:

so I should put 167 ohms in series for each segment?

Click to expand...

Not 167Ω. Use whatever value you come up with in the equation shown above but you are correct that you will need one resistor per segment.

CAPT Bligh said:

Is there anything else that should be included on this simple circuit--capacitors, shunt resistors, other things I'm too ignorant to know about?

Click to expand...

Not really. I'm not certain how you are going to wire up the displays to produce jibberish unless the switches are correctly set but for the LED display portion, nothing is needed excpet for the power source and the current limiting resistors

CAPT Bligh said:

If you understand what I'm trying to do, is there anything that could improve the "coolness" factor of this, without making it too complicated (don't want people frustrated out in the middle of the woods on a multi-stage)?

Click to expand...

Coolness factor wasn't on the datasheet you linked too. But seriously, nothing is coming to mind right away. Maybe someone else will have an idea.

vne147,
Many thanks! Regarding the batteries, I took a look at the ratings of some common brands to get an idea of how long they would last. If I assume that, on average, half the segments will be lit when the lid is opened (I'll have a limit switch on the lid), then at 25mA per segment this thing would draw 21 x 0.025 = 0.525A. For a battery like the standard Energizer 9V, the datasheet shows about 300mAh at 500mA discharge. So I think that means it would last for less than an hour of operation.

If I stacked four alkaline AA's, each battery would supply 1/4 of the current, so they would be delivering about 125mA each. According to the datasheet, they are upwards of 2000mAh at that current, so I would get 16 hours of operation.

AA lithiums claim to have 3000mAh indepent of discharge rate. That would give me 24 hours.

Four alkaline D cells, at about 15000mAh each, would give 120 hours. Probably excessive, but just bounding the problem here.

Not asking you to check my math (well, you can if you want), but am I thinking about this correctly?

CB

CAPT Bligh said:

I took a look at the ratings of some common brands to get an idea of how long they would last. If I assume that, on average, half the segments will be lit when the lid is opened (I'll have a limit switch on the lid), then at 25mA per segment this thing would draw 21 x 0.025 = 0.525A. For a battery like the standard Energizer 9V, the datasheet shows about 300mAh at 500mA discharge. So I think that means it would last for less than an hour of operation.

Click to expand...

I didn't double check your math but it sounds about right and your method is correct. That's why I only suggested the 9V with the caveat if you weren't going to leave the display powered all the time. If each user only spends a few minutes playing with the thing, an hour might be OK but it's your call. You'll know better than me how many people will need to use it and for how long.

EDIT: Nevermind the red highlighted idea. This would work just fine for individual LEDs but I realized after I wrote it I'm not sure how it can be done with a common cathode 7-segment display.

One thing I didn't mention is that if you decide to use a 9V, you can get away stacking 2 displays in series. So you would have 3 parallel banks of 2 displays in series. This would cut the current requirement in half and also half the number of resistors you'll need.

CAPT Bligh said:

If I stacked four alkaline AA's, each battery would supply 1/4 of the current, so they would be delivering about 125mA each.

Click to expand...

When batteries are arranged in series, ALL the current required for the circuit will come from every battery. To have a battery supply only a portion of the operating current the batteries must be arranged in parallel but then the voltages don't add up. You can add up volatges or you can spread out the current draw but unfortunately you can't have both.

For one thing, you could have a momentary switch to power the whole thing. A momentary switch is one that returns when you let go. That way, anyone wanting to look at the display holds the switch on while looking at the display, but as soon as they let go it stops taking power at all.

vne147 said:

When batteries are arranged in series, ALL the current required for the circuit will come from every battery. To have a battery supply only a portion of the operating current the batteries must be arranged in parallel but then the voltages don't add up. You can add up volatges or you can spread out the current draw but unfortunately you can't have both.

Click to expand...

Doh! I won't embarrass myself further by explaining why I should have known that...

I think I'll go with a 4D holder. A couple dollars more for the power, but lots of confidence that it will last a while if I can't get out and check up on it regularly.

Many thanks for the responses.

CB

Diver300 said:

For one thing, you could have a momentary switch to power the whole thing. A momentary switch is one that returns when you let go. That way, anyone wanting to look at the display holds the switch on while looking at the display, but as soon as they let go it stops taking power at all.

Click to expand...

Thanks, Diver. I think with the limit switch on the lid and a big 4D power pack, I should be OK even if they have to think a while. I don't expect more than 20 or so finders per year.

What I'm really stumped on now is figuring out how to make sure they reset the switches to something other than the right answer when they're done. Otherwise, the next finder will be looking right at the answer! I'm thinking about a switch cover that spans all the switches, and it will only close when they're all down, or something like that.

How would you feel about a microcontroller solution? You could have it reset everytime the lid was closed and then have it create a new random pattern everytime the lid is re-opened.

vne147 said:

How would you feel about a microcontroller solution? You could have it reset everytime the lid was closed and then have it create a new random pattern everytime the lid is re-opened.

Click to expand...

I did think about that (but I would have no idea about how to actually implement it). The problem is, there would be 128 different solutions for the seven switch positions. If the solution is random, they'll have to just try every possible combination until they hit paydirt--and I suspect they'll just get frustrated and give up. So I was planning on giving them a clue about the required switch positions on the cache web page--which would require the solution to be singular.

That being said, a random solution generator would be a lot more interesting to build

Any ideas on how I could have a set of possible solutions, and clue them in to it?

CAPT Bligh said:

I did think about that (but I would have no idea about how to actually implement it)

Click to expand...

Micro-controllers can be daunting at first especially if you've never done anything with them before. If you decide to go with the micro-controller idea I would be willing to help you out. I can deisgn a circuit and post the schematic, write the program, burn it onto a micro-controller, and ship it to you. All you would have to do it assemble the project according to the schematic. I'd only ask that you pay for the micro-controller and shipping. I'm not sure where you live but it would probably come out to less than $10.

CAPT Bligh said:

The problem is, there would be 128 different solutions for the seven switch positions. If the solution is random, they'll have to just try every possible combination until they hit paydirt--and I suspect they'll just get frustrated and give up. So I was planning on giving them a clue about the required switch positions on the cache web page--which would require the solution to be singular.

That being said, a random solution generator would be a lot more interesting to build

Any ideas on how I could have a set of possible solutions, and clue them in to it?

Click to expand...

One idea that comes to mind right away would be for you to use less switches. 4, 5 or maybe even 6 switches might be less exhaustive to find the solution. I'm not sure how we would be able to give a hint to the user about the solution. I'll think on it for a bit. Ideally we would want to minimize frustration while maximizing coolness factor.

Don't use batteries

For Geocaching I would forget the batteries and find a windup radio or flashlight you can pilfer the parts from.

So when the user opens the box they can use the crank to power the system up.

No need to make frequent trips to the location to check batteries. And you don't need to worry about the batteries leaking, or being dead when someone gets to it.

Maybe you can do this for version 2.0 since using batteries would get the project working faster.

Good luck. I enjoy geocaching.

my 2 cents,

They could use there own battery.

could have a hole for a 9V battery to slide down into, with two small springs to make the connection (use the wight of the battery to hold the connection),, Then use a rectifying bridge (4 diodes) so that the polarity of the battery won't matter.

You could use DPDT or 3PDT switches, And use diodes and resistors to power differ segments. If I could just upload a picture/diagram. it would explain everything i am trying to say.

If you wire in a blinking LED (in series), it will also blink the segments. (could act as a hint )




There is always a Microcontroller (would need a 5Vdc reg, and a 10000uF cap), it could do some cool stuff.
The closer they get to the right combo the faster the lights will blink per digit.
With a uController you could even use a LCD.

badscr said:

There is always a Microcontroller (would need a 5Vdc reg, and a 10000uF cap), it could do some cool stuff.

Click to expand...

Is this a typo?

CAPT Bligh said:

That being said, a random solution generator would be a lot more interesting to build

Any ideas on how I could have a set of possible solutions, and clue them in to it?

Click to expand...

I did have an idea about how you could clue in the user as to the random solution. The solution will be a combination of switches some of which are on and some of which are off. So the solution is essentially a binary number. For example, if there are 6 switches and the random solution is for switches 1 & 3 to be on and the rest to be off, then the binary equivalent would be 000101. The microcontroller could be programmed to flash the binary equivalent of the solution on the display for a second or two when the lid is first opened and then immediately start displaying constantly changing random numbers or jibberish until the user sets the switches for the correct solution. There could also be a green LED that flashes or something like that to indicate when the correct combination has been achieved.

No its not a typo.
It is more for momentary power loss, so that the PIC wouldn't reset.
Or even a 1 FARAD 5.5 VOLT "SUPER CAP" for $3.50 (www allelectronics com CAT# CBC-17)



A hand-crank for a power supply may get broken. But I do like the idea.
Them providing their own battery would be cheaper.

badscr said:

No its not a typo.
It is more for momentary power loss, so that the PIC wouldn't reset.
Or even a 1 FARAD 5.5 VOLT "SUPER CAP" for $3.50 (www allelectronics com CAT# CBC-17)

Click to expand...

A 1F cap seems a little excessive to me and a 5V regulator isn't a requirement either. Most PICs are perfectly happy operating off of unregulated batteries with much smaller than 1F smoothing caps. I recently built a 9 LED sequencer with a 16F627A that was powered by two unregulated AAAs and a couple of small (10u and 100n if I remember correctly) smoothing caps. It worked just fine. If the circuit did momentarily lose power (not sure why that would happen) it would reset and just pick a new random solution.

surprised a solar cell hasn't been mentioned.
An LCD display (robed from a digital watch would be a possibility.
I was thinking along the lines of a PIC as well.
Using one monetary push-button, have the first digit go from 0 to 9. when desired digit is displayed the user pushes the button. If correct the next digit is displayed. The digits change every 5 seconds or so so to count from 0 to 9 would take 45 seconds. If the wrong digit is selected the wrong digit keeps changing. Upon a correct digit THAT digit remains ON and the next digit starts changing. Continue on until the correct 3 or 4 digits are displayed. Only the digit being guessed is displayed as well as correct digits.When done just place back into the Geocache box. The unit automatically shuts off. How long will a watch battery last? plus add a solar charger. The user battery is nice but only if the user remembers to bring a good battery.

I did think about the solar idea but I assumed that this thing would be kind of hidden away in a tree stump or something like that while waiting to be found. Bottom line is I didn't think the sun would be hitting it. A rechargeable watch battery might have a high enough capacity to keep the thing powered long enough for the user to find the solution but I'm not sure if it could supply the required current which could be as high as 100 - 150 mA depending on the segment current of the 7 segment displays. If he were going to go with the solar cell idea, I'd think 2 or 3 rechargebale AAAs might work better.

Could eliminate the switches by using mTouch capacitive touch sensing.

go with a watch display. very low power requirements.
upon finding, the user has the unit out in the sun for several miniutes. I estimate at least 10 miniutes each time.
just get hold of a discarded digital watch and use the display.
one N.O. button for selecting desired digit guess. very little power usage.

Hey folks, I really appreciate all the feedback.

I'm enjoying thinking about all the ideas offered. I don't think the solar option will work well, because as vne157 guessed, the container will be hidden and won't get much if any sunlight. I'm not really concerned about power capacity, because there will be lots of room available in the container and I'm happy just using a four pack of D cells for practically unlimited "on" time.

I am having to restrain myself from jumping on the more complex ideas because I don't want to frustrate people in the field. The point of the hobby is the hunt and if they are able to find the box, then I don't want them to be prevented from getting to the next stage because they can't figure out the settings. And sometimes they'll be standing in the rain, or it will be really hot, or they'll have small children with them, etc, so I want to make it more enjoyable/easy than satisfying-to-solve-but-hard. So it's a fine line I'm trying to walk--just hard enough so that's it's not a giveaway, but not too hard. Maybe a 2 or 3 on a scale of 10. So I guess I'm looking more for electronic "bells and whistles" than difficulty. I thought about have one extra display unit to show how many switches were set correctly at any given time, but then I realized that would be TOO easy--they would just have to cycle each switch and see if the count went up or down.

Maybe some kind of "victory dance" of leds surrounding the display when the solution is found? Maybe something that would play an audio clip (the theme of the cache is "Snoopy and the Red Baron--Snoopy has been shot down behind enemy lines and is being held hostage; the cacher has to get the coords of the military outpost and go rescue him--so it could play the "Snoopy vs. the Red Baron" song ), maybe the display could be sympathetic and flash the correct settings in binary after a certain number of failed attempts...?

But please keep all the ideas coming. I'm collecting them for future caches that I want to make intentionally hard