Led targets project for paint ball

[ericwalker]

I am designing a target board for my paint ball game. I plan on using an ambient light sensor to actuate a small led light strip to either come on or go off when the light sensor is covered with paint. I have a light sensor made by Vishay (TEPT5700) that may work. The tech at Vishay stated that it only needs light to work, though the documentation sent to me stated that a load resistor of 10 ohms will produce 3-5 volts. I do not want to damage it so I am asking if anyone may know the size load resistor I should be using. The led lights work off of 12v and produce 24watts of power. I used an online calculator to get the other 2 answers - 2 amps and 6 ohm load resistor. Is this correct? I am not an engineer, I know enough, not even, to be dangerous..thanks for the help..

 

[dougy83]

The datasheet is here https://www.electro-tech-online.com/custompdfs/2013/08/tept5700.pdf

The device will allow a very small current to pass when it is illuminated. When it is covered, less current will pass. You will need to invert and buffer this current with some device(s) (e.g. some transistors) to drive the LEDs

Your calculation of 6 ohms is not relevant to your LEDs. Most likely your LED strip has the appropriate resistors installed to allow it to be connected directly to 12VDC. You should look at the data that applies to your particular LED strip.

 

[ericwalker]

Hello, thanks. I understand that the transistor will "amplify" the current. This product then will not be damaged by 12vdc source? It will actually "hold back" on allowing current to the leds, hence a transistor. Can you recommend the correct transistor for this project, and the other parts that will allow this project to be successful? And is it as simple as soldering the parts inline, or should I consider a small circuit board? Again, I am using a 12vdc battery to power upto 64 of these targets - one light sensor with a 6" led light strip. The ambient light sensor will act as the switch, allowing current to flow or not to the led lights.

Also, the led lights are offered by Hitlights. The tech there (electrical engineer student) said that each 12" strip produces 1.5 watts, and each 16' strip pulls 2 amps, does this help?

 

[alec_t]

The LED strip spec is unclear/inconsistent. In post #1 it's "12V 24W" whereas in post #3 it's a "6 inch strip" or a "12 inch 1.5W strip" or a "16ft 2A strip"

 

[ericwalker]

Hello, sorry about confusion:

The led lights strip is 16 feet long - it takes 12vdc and consumes 24watts. In my application I plan on cutting the 16' long strip into 6" strips. Each 6" strip will have an ambient sensor connected to it. I plan on making about 64 of these circular targets - 16 targets per a 4x4' wood panel. All 4 panels will be connected together and one 12vdc battery will power them. I need the correct transistor to solder inline with each sensor, allowing the 6" strip to make the maximum luminance possible..

 

[alec_t]

I plan on cutting the 16' long strip into 6" strips.

Is it designed to enable that to be done? The way the LEDs are presently grouped into serial/parallel strings is key.

 

[ericwalker]

Yes, the led strips are cuttable every 3"..pretty cool product and they still work off 12v even at that size. I planned on parallel, unless you offer a different opinion?

 

[alec_t]

Then I infer that 3" strips must already be in parallel, whatever the interconnections within that 3".
Unless you have already bought them or can buy them very cheaply the TEPT5700 is less than ideal for this task.

the documentation sent to me stated that a load resistor of 10 ohms will produce 3-5 volts

Something wrong there. Fig 3 of the datasheet indicates that with a collector-emitter voltage of ~ 3-5V the current at 20 lux would be typically ~ 10μA, so a 10Ω load would produce a voltage drop of only 100μV. Do you know the ambient light level at the targets?

 

[ericwalker]

The ambient light at target will vary between sunlight to florescent lighting. In other words, the booth can be installed indoor or outdoors. Is there another light sensor that you can recommend?
I already have the led light strips, was using them because they are readily available. I had another light in mind - digikey -part #350-2126-ND, this might be a better option? Thanks

 

[alec_t]

Is there another light sensor that you can recommend?

I'd use a small CdS cell, such as a VT90N1. This changes resistance dramatically according to light level so is easier to apply. For your purpose you don't need a high-speed photosensor.

digikey -part #350-2126-ND, this might be a better option?

Don't think so. It's expensive and only a 5mm diam light source, and probably not daylight-visible. Have you checked that your LED strips are clearly visible in sunlight?

 

[ericwalker]

I looked at Digi-key, which is the product I need to look at for this application? What other circuitry would be needed for this idea to work with the led light strip. Yes, the led strips are visible in day/sunlight.

 

[alec_t]

Try searching for 'VT90N1' (as mentioned in post #10) or 'photoresistor' or 'light sensitive resistor'.

What other circuitry would be needed

I'll have a think and post a schematic.

 

[ericwalker]

Thanks, any help on that would be great. I will look up the product. If you do post a schematic please "dumb" it down a bit, I am still learning, obviously.

 

[alec_t]

If you do post a schematic please "dumb" it down a bit, I am still learning, obviously.

Any circuit I suggest is likely to need a certain amount of adjustment to set it up to be suitable for LEDs whose full specification is not known, for sensors etc with manufacturing tolerances, and to cope with varying ambient light levels. Do you have a Digital Multi-Meter (DMM)? Have you previously built any circuits?
Can you post a link to the datasheet/specification for the LED strip?

 

[ericwalker]

I have a dmm. I have removed and added circuits via soldering and de-soldering irons...I have also wired prebuilt boards - remote controls.. The closest I have come to building a circuit board is a radio shack kit - very small for led light ironically. if I have a schematic that explains what to put where I can get it done.

 

[alec_t]

Ok. Here's a suggested circuit for 1 target. It may or may not work without modification depending on which photocell you can source cheaply, the precise properties of your LED strips and the range of ambient light levels. I have assumed that a 6" LED strip requires 750mW of power, hence a drive current of ~ 63mA.
I strongly recommend you buy only enough components to build and test one of these initially, before committing yourself to major expense for 64 targets. If any component proves difficult to source we can consider alternatives.

The circuit uses an LM324, which is a quad opamp IC; so one '324 could be used with 4 targets providing they are placed reaasonably close together. If your targets have to be far apart then it would be better to use individual single-opamp ICs instead of the '324. To cater for various ambient light levels one photocell (CellRef) and a 100k potentiometer (Uref) are used to provide a semi-auto-changing reference voltage which can be used as the reference (ref) for all 64 targets. Each target has its own 100k trimmer to cater for the wide sensitivity tolerance typical of photocells.
The photocells would need to be protected from paint contamination by transparent plastic windows/sheet, and if a '324 is shared by 4 targets each cell would be remote from the '324 so would need twisted-pair or screened cable to connect it to the circuit.

 

[ericwalker]

This looks great, a bit greek to me, but great, thanks. The booth is made up of 4x4 panels for travel purposes. So the back wall, I will have 4 - 4x4' panels - with 64 targets that's 16 per panel - spaced equally apart that is a space of roughly a 12"x16"space between targets in each directions - vertical and horizontal. Is this space to much for the LM324?

Just to confirm when shopping for parts: Most likely will place an order with digikey: I need the following:

Photo Cell
LM324 or single-opamp ic (part name?)
100K Potentiometer
100k Trimmer

What else? What size circuit board would I need? if any?

The other question is - does kind of power is this schematic requiring? I was hoping to power off a single 12vdc battery. What amp hour should I consider if I want to power all together or split in to 2 sides - 32 and 32?

 

[alec_t]

That target spacing looks ok for 4 targets sharing an LM324.
Digikey list many photoconductive cells. The cheapest seems to be the PDV8103, which should be fine.

What else?

One 0.1μF ceramic capacitor, not shown in the schematic, per LM324 (but if noise pick-up on circuit wiring proved to be a problem you might need 6 per group of 4 targets), hook-up wire, housings for the project boards, terminal blocks, solder, resistors (1/8W or 1/4W rating), strip-board (circuit board).

What size circuit board would I need?

I would suggest one circuit board ~ 3" x 3" per group of 4 targets.

I was hoping to power off a single 12vdc battery.

Ok.

What amp hour should I consider

Depends how long you will be operating the system between charges. Your LEDs need 24W, which equates to 2A. The circuits draw ~8mA per target, amounting to ~0.5A for 64 targets. So in theory a 10Ah battery (for example) would have a life of 10/(2+0.5) = 4hrs. In practice you wouldn't want to run the battery flat so you'd probably get only half that, i.e. 2hrs. I guess a 10Ah battery would therefore be inadequate?

 

[ericwalker]

OK, here are some additional questions:

The LM324 - there are thousands, which one should I use - what MHz, other filters? I am looking at digikey...
Noise pickup - please explain? Also, is it 9 capacitors per strip board in order to operate 4 targets?
The Capacitors: Again, is it general purpose: I used a filter to find the o.1uf and came up with thousands - do I have to worry about volt ratings and other criteria.
The 100K trimmer - again, many to choose from. I do not know enough about how to filter for the right one - what does 100k mean?
The 100K Potentiometer - I am confused: looking up on digikey I see, after filtering that the description says 100k trimmer, so are these 2 products really the same?
The resistors - what type? Many options want to have the right one.
The terminal blocks - what type - board to wire?
The Strip-board - What Type?

The power supply - the system needs to be powered upto 5 hours. I can either power all 64, or use 2 batteries - one per 32? I already own 18amphr batteries - I would use one per 32 targets to ensure the system does not fail.

Sorry for all the questions, I admit, I am way out of my league here and I appreciate your help in "leading" me through this. Once I have the "correct" supplies I will be asking the proper way to read your schematic and then how to solder them correctly - in the right direction.

 

[alec_t]

All components are the 'through-hole' mounting type.

The LM324 - there are thousands, which one should I use

Search 'LM324N'. You get 12 hits. Digikey part no. 296-1391-5-ND would be my choice on cost and minimum order quantity grounds.

Noise pickup - please explain?

Lengths of wire from cells to circuit board act as aerials and pick up radio waves. Capacitors can be used to suppress the unwanted effects of these waves. One cap per strip board is essential and is connected directly across the power supply pins of the LM324. Hopefully the other caps would not be needed, so you should defer buying them until you have tested a prototype target. The caps are the ceramic type, radial, voltage rating >20V. (The higher the voltage the more expensive the cap).
Trimmers are single-turn, top adjustment, carbon.
Potentiometer is single-turn, top-adjustment, carbon, >= 0.05W.
Resistors: carbon, axial lead, >= 1/8W, 10% tolerance.
Terminal blocks: barrier blocks, pcb mounting, board to wire, >= 0.5A
Stripboard: Veroboard.
Those two batteries should cope.