I need help with basic electronics

[Michael Kellough]

Flashing light? Bells and whistles?
Anyone knowledgeable in the area of basic electronics?

I’ve been hired to make some props for a corporate
convention. These things are token receivers to visually
display the votes different performance regions receive
from the convention attendees. (People will drop tokens
into a clear acrylic tube.) The design calls for a light to
come on when the token is inserted into the chute. The
problem is that the light should stay on for a couple seconds
so I need some kind of delayed off thingy. Or, the light should
flash for a couple seconds, though I don’t have any indication
from the client at what frequency. Another option is for the light
to come on when a person gets very close to the token receiver
and then turn off when they leave. This would call for a low and/or
adjustable proximity sensor. (Or a simple floor mat switch) The
light source will probably be 12v 20 watt halogen or  even lower
voltage 3 watt LEDs. I’m leaning toward the halogens as I don’t know
the ambient light level of the show area and I can use conventional
electrical devices to control power to the 12v transformers.

Any suggestions?

The TokenTube.pdf shows the basic design which will be fabricated
of cast clear acrylic. The light will be inside the horizontal tube
shining up though the flat acrylic to illuminate graphics applied to
the flat surface. The things will be mounted to the wall of the display
booth or mounted on a post. In either case the client has approved
the use of a power cord running from the prop so I don’t have to use
batteries.

 

[Dan Lyke]

Do a search for "time delay relay". I can't speak to 'em, other than that one of my gigs was to build a custom motor control system that was prototyped in China with time delay relays, and they threw the things around like they were candy.

Close a circuit, they turn on for a fixed amount of time, then turn off. Available in different voltage ranges, it should be fairly easy to find ones that switch 15 amps, which should be more than enough for some simple lights.

You could go all complex with microcontrollers and distance sensors and all of that, get your parts cost down to a few pennies for a few tens of thousands of dollars of R&D costs, but it sounds like something that's cheap to do one-off and simply functional is a much better solution.

 

[Michael Kellough]

Thanks for the tip Dan. That sounds like the thing I need.

R&D would be fun for a gag like this but not enough time or money, as usual.

 

[Rick Christopherson]

I apologize for any potential confusion this may cause, but I have chosen to not participate on this forum.

 

[Michael Kellough]

Hi Rick, thanks for the very interesting reply.

Let me paraphrase to see if I understand how this would work.

The optical interrupt sensor is normally open. It only closes when the coin is inserted and that split second closed is enough for a bit of charge to develop in the capacitor. Then, "When the switch (sensor) is opened after the token has passed, the capacitor discharges through the LEDs and resistor slowly."

So, power is supplied first to the switch (in series) and then to the capacitor and resister (both in parallel) and finally to the LEDs (in series).

Assuming I got the basics right, can you answer a couple of other questions?

Optical sensor design.
I guess with the sensor in the photo you kindly posted that the coin must pass between the two protrusions opposite the leads? Could this sensor be cut in two to gain more flexibility in mounting. Or, are there other designs? Do you know a web site for comparing these things? Also, why is there an extra pair of leads in this sensor?

I don't know if the art director is going to approve putting this kind of sensor in the top of his clear acrylic design. Do you know of a proximity sensor type device that could be more discretely placed?

LEDs.
Do you know how sensitive LEDs are to voltage? If I pulled LEDs from a typical flashlight and then gave them 12 or 24 volts would they blow? Also, I get the impression that the 20 watt halogen lamps I might need wouldn't get enough power from the capacitor, is that right?

Thanks again Rick!

 

[SaltyDog]

To make your time delay relay, if you can't find what you want.

do a search on 555 timer.

there are scads of application notes out there for it, it has a One shot mode where you trigger it, (from the optical switch above)
and then it has a programmable output pulse width that you can vary from usec to secs.

S

 

[Dan Lyke]

I guess with the sensor in the photo you kindly posted that the coin must pass between the two protrusions opposite the leads? Could this sensor be cut in two to gain more flexibility in mounting.

The optical sensors I've played with have an IR (or visible, but usually IR) LED on one side and a phototransistor on the other. Break the light and the phototransistor stops conducting. The better ones modulate the LED so that the phototransistor is less sensitive to light noise in the general environment. Obviously there are alignment and attenuation issues as you get things further apart, but, yes.

At more packaged level, RadioShack sells those boxes with which put a reflector on one side of the store front and bounce a light off of it across tens of feet and buzz when you walk in the store, they probably even have such a thing that you can plug 110v into. If they don't, a home automation supplier mgiht.

Do you know how sensitive LEDs are to voltage? If I pulled LEDs from a typical flashlight and then gave them 12 or 24 volts would they blow?

The trick with LEDs is giving them a fixed amount of current, which you usually do by giving them a fairly precise voltage. So, yeah, if you give them 12 or 24 volts without an appropriate resistor to give them the current they expect, they'll blow. There are various LED calculators out there on the web, give them the voltage you've got, the current the LED expects (usually from the LED spec sheet), they'll give you the appropriate resistor values.

 

[Michael Kellough]

Thanks guys, SaltyDog and Rick. If my head doesn't explode first I might learn enough to get this done. A good example of how little I know is asking if the optical emitter and receiver could be further separated and then wondering what those extra wires are for. Duh.

By the way, what is the third wire for? Is that on the receiver side and is that the wire that would go on to the capacitor in Rick's initial scheme?

If I start the 555 timer with some kind of switch I'll need a relay suitable for the volt/current the lamps will require and one or more power supplies for the components? I suppose the 555 and relay could be battery operated and housed in the horizontal lamp tube and the transformer (for 12v halogen) on the floor?

If I used a 555 timer could I start it with something as simple as a momentary microswitch that would be actuated by a lever in the path of the token? If so I guess the switchmat on the floor is also still a viable option? (Are there 120v capable 555s to work directly with the switchmat?) I'm not uninterested in the sensor suggestions just trying to keep the appearance of the prop clean by hiding the electronics as much as possible.

I hope my ignorance isn't too trying on your patience.

 

[Michael Kellough]

Thanks for the suggestions and tips Dan. If it wasn't for you guys on the forum I'd be trolling Radio Shacks untill I found someone knowledgable enough to help me. Actually I might have to do that anyway but at least I won't be quite so clueless when I get there.

 

[Dave Ronyak]

Michael,

How are you planning to guide the tokens so they will assuredly pass through your sensor?  A narrow tube?  A funnel in a larger tube positioned just above the sensor?

And for Rick C., would a capacitive sensor be effective over a broader "target area" thereby mitigating the need to ensure that the tokens follow a narrow trajectory?

 

[Michael Kellough]

Hi Dave, if you zoom in to the top of the TokenTube.pdf  (in the original post) you'll see a slot across the front of a 2" OD acrylic tube. The ID of the tube is 1 5/8" and the token diameter is 1 1/2" so the leading edge of the token will have to project about 3/4 of the way across the tube before it can fall in. A tiny discreet lever arm could be  mounted on the back wall of the tube so the leading edge of the token should hit the lever with enough force to momentarily close the circuit. That part I can handle but I've got a lot to learn to be able to electronically sustain the resulting light for long enough to satisfy the client.

I'm not sure I can be certain that the coins will reliably trip an optical switch since the parts of the switch would have to be on opposite sides of the tube near the top and the token will be presented edge on, maybe too small an area. I also don't want to put such obvious components right in the area people will be looking at. Of the sensors I've learned about here I'm more inclined to use a proximity sensor either to detect the token when inserted or the body of a person approaching the prop.

At least I've learned that the components are readilly available and there are people for whom this is relatively simple.

 

[Louism]

Look around for some coin-op venders like Happ Controls or Peach State. They should have whatever you need in the way of coin mechanism and elecronics to match.