Thread: Animated Prop Control with PLC

Tweet

Hello to all. I have several animated props: Grape Lifter, Popup Zombie, and Grave Digger to name a few. My plan is this year to start to automate my display using a PLC. All of these props have a provision for a footpad. I disassembled a Spirit Halloween footpad and found the trigger mechanism. When checking with a good VOM (fluke 87) at rest state I show an open circuit. As I put pressure on the trigger I get a reading from .1 to 1.3 Meg for less than .5 sec then back to open. This triggers the prop. I tried to just short the wires together momentarily to trigger the prop, however this method is not reliable. I was wondering if anyone has had experience with using the footpad input to trigger this type of prop. I realize that I could do a quicker trigger with a PLC using a momentary contact in the ladder and limiting the on time but, dose this circuit need some resistance in it to operate correctly without damaging the prop?

I'm not familiar with the Spirit footpads, but the idea that an idle condition is "open", and an active condition is "<0.5sec pulse of between 100k and 1.3M" sounds dubious. ("open" and 1.3M are way to close to one another) Where were you measuring, and was the circuit powered up? Was this foot trigger a separate device, or was it part of the prop?

One option (that would get around figuring out exactly how it works) would be to wire your PLC's relay across the actual switch that gets closed when someone steps on the mat. That's guaranteed to be just a switch, whereas the interface between the mat (as a whole) and the rest of the prop is unknown.

- Hook

Hooked_on_Scares thanks for the reply. The footpad is a separate piece, below are some picks of the dissected footpad. There is a foot pad port on the prop, when using it you are supposed to set the 3pos on-off-footpad switch to footpad position. Last Halloween when the props were all set to footpad position when you step on the pad the props would activate. Also, if you would barely move the pad (no downward pressure) or there was a loud noise such as a car horn honking this would activate the props. My thinking may be wrong as to this being a resistance triggered device maybe it is capacitive.

I wrote Tekky Toys with a similar question and here was the reply:
“I'm glad you are enjoying our products. Unfortunately, we are not provided with such internal details about the wiring of a product, therefore I cannot give you any tips on how to trigger the item the way you want to. We do not encourage tampering with the wiring in any way, shape, or form. The items are very sensitive and can very easily be broken, and if that happens we will not be able to assist you by sending any new parts. I just want to warn you that tampering with the wires in these items are very risky.”

Obviously this person doesn’t understand my determination.

Got it it's a Piezoelectric sensor. I just need to figure out the voltage thresholds to trigger the device

I concur on the piezo. But that doesn't surprise me at all. There's very few mfg who'd be interested in helping a hacker. I haven't worked much with piezos, but I think they'll just put out a short spike of voltage, when struck. So, that's probably what the receiving circuit is looking for. If you're able, see if you can figure out what the receiving circuit looks like (chip-wise). I guess they use piezos instead of switches, because they'd be more resilient.

- Hook

Hook give me some time to investigate life is getting in the way of my projects.

Picture #1 Is the base of the Grave Digger (which looks curiously like the base of the large floating broom) anyway all screws must be removed. Don't worry about marking the base you can use the sticker on the footpad port to orientate.

Picture #2 What are those white bags? Descant bags.

Picture #3 Carefully remove the hot melt glue from the one support screw and the one corner hot melted to the base.

Picture #4 The circuit board is now free to manipulate.

Picture #5 One chip allot of Capacitors, Resisters, and Transistors

Hook here is a pictorial of the circuit using poor man's autocad (MS Word). As you can see by the previous post the connections were made on the cheep so instead of being connected neatly on the edge, they were direct connected to components. I just put the first component in line to see if that was enough info for you to make a conclusion. There are also circuit traces that go else ware at the connection point.
My thought was to apply 1 volt DC across the connector do you see a problem with doing that?

Frequency response of a piezoelectric sensor; output voltage vs applied force
A detailed model includes the effects of the sensor's mechanical construction and other non-idealities. The inductance Lm is due to the seismic mass and inertia of the sensor itself. Ce is inversely proportional to the mechanical elasticity of the sensor. C0 represents the static capacitance of the transducer, resulting from an inertial mass of infinite size. Ri is the insulation leakage resistance of the transducer element. If the sensor is connected to a load resistance, this also acts in parallel with the insulation resistance, both increasing the high-pass cutoff frequency.
In the flat region, the sensor can be modeled as a voltage source in series with the sensor's capacitance or a charge source in parallel with the capacitance
For use as a sensor, the flat region of the frequency response plot is typically used, between the high-pass cutoff and the resonant peak. The load and leakage resistance need to be large enough that low frequencies of interest are not lost. A simplified equivalent circuit model can be used in this region, in which Cs represents the capacitance of the sensor surface itself, determined by the standard formula for capacitance of parallel plates. It can also be modeled as a charge source in parallel with the source capacitance, with the charge directly proportional to the applied force, as above.

Hmm... well, that transistor is a PNP, and the piezo is connected to the emitter. Which makes me wonder if there are two piezos we're talking about? Piezos can be used to convert vibrations to voltage, or voltage to vibrations (sound). Given the diagram you showed has the piezo connected to the emitter of the transistor, it seems like this circuit would be to drive the piezo, rather than take input from it.

I think the most straightforward way to hack this is to get at the 'Try Me' switch I see in the 3rd post. I know it'd be nicer to get the thing to be triggered through the jack. But I wouldn't be comfortable suggesting any voltage to pipe in there, until I understood how the whole receiving circuit worked. (since I don't know that much about piezos)

- Hook

Hook, thanks for your help, I had a friend come over to help with my project. After doing some testing with a signal generator and a o-scope we came up with this circuit to replace the Step Here pads.



The 1meg resister is needed to discharge the cap and the voltage is pulsed with a contact to trigger the prop.