ATTiny85V + TIP122 transistor to explode giant balloons


I’m working on a device that will detect the vibration of a projectile’s impact on a giant latex balloon and then allow sufficient current to pass through a heating element to explode the balloon.

In truth, I have managed to create a working prototype, firstly using an Arduino UNO, and then an Arduino NANO (in an attempt to reduce the physical size). In both cases, the circuit was powered by 9 volts from 6xAA batteries. A piezo disc detected the vibration of impact, the Arduino switched an output pin HIGH connected to the base of the TIP122 which allowed current from the 6xAA to flow through an Estes Solar Rocket Starter as the heating element. Worked well. You can see the result in action here.

In order to shrink/improve the device, even more, I am trying to replace the Arduino NANO with an ATTiny85V and the Rocket Starter with a Remote Control Engine Glow Plug but I am having trouble and hope for some advice.

The ATTiny85V is being powered by 2xAA batteries.
The Glow Plug can use a 1.5 - 3.0V power supply and draws around 2.5 amps from that supply for the 1.5 seconds that it is switched on.

I have attached a schematic of my test circuit. It doesn’t include the piezo sensor because that ‘side’ of the circuit works as planned. It only includes the ‘ignition’ side of the circuit which I can’t get working.

PB3 on the ATTiny85V switches HIGH when the push button switch is activated. In turn, PB2 is then switched HIGH supplying current to the base of the TIP122 transistor, allowing the flow of current through the Glow Plug Igniter for 1.5 seconds.
The 3V supply on the schematic is provided by a pair of AA batteries.

Problem is, I can’t get sufficient current to flow through the Glow Plug and I don’t understand why.

I have used various size resistors at R2 without success. I have even removed R2 all together (bad option, I have learned) but still the Glow Plug would not glow.

Am I asking too much of the TIP122? Is there a better option?
The Glow Plug glows immediately if connected directly to even a single 1.5V AA battery, but not if connected via the transistor.



Moderator edit: PDF. Ugh.

attiny85v-pushbutton-test.pdf (403 KB)

I have used various size resistors at R2 without success.

I'm thinking around 1K.

...And I think you need more voltage (another battery or two).

Does the glow-plug glow when directly connected to the batteries?

Do you have a multi-meter?

You're going to get some voltage drop from the batteries when drawing 2.5A. But I don't know how much, so if you can measure the battery voltage with the glow-plug attached, do that.

Then you're also getting some voltage drop across the transistor and I suspect that's the real problem.

Low Collector−Emitter Saturation Voltage −
VCE(sat) = 2.0 Vdc (Max) @ IC = 3.0 Adc
= 4.0 Vdc (Max) @ IC = 5.0 Adc

...Is the glow plug fast enough? I assume it takes a fraction of a second before it starts to glow?

I think your problem is the on-state voltage drop across the TIP122 - Vce(sat) is 2V @ 3A. It is an obsolete part and a totally inappropriate choice for a 3v system.

Try an IRF3708 - assuming your batteries can happily supply that current - connect drain in place of collector on your TIP122, source to ground, gate through 100 ohm resistor to pin of the microcontroller, and 10k resistor from that pin to ground.

I never tried it but what about tantalum caps? They should burn and/or explode when powered by reverse voltage - maybe it can be used as a simple fuse to pop the baloon?

I never tried it but what about tantalum caps? They should burn and/or explode when powered by reverse voltage - maybe it can be used as a simple fuse to pop the baloon?

The behavior isn't reliable enough, I've played with blowing up tants. They really don't live up to their reputation IMO.

If you have enough current, you can use tufts of steel wool (it's fairly impressive)

But no matter what you use, getting it hot is a tall order with only 1 volt left after the voltage drop across an obsolete darlington transistor like the TIP122!


Have you considered using the special glow wire they use in e-cigarettes. I have some here, and I have refurbished used (burned out) glow wire in these components. The e-cigarette works on a 18650 battery witch provides 3.7V. To refurbish the component you need several winding's to prevent the wire to burn out right away. So if you used less winding's then you should be ok for your 1Volt. Do a search on e-bay for Kantal wire, or visit your local e-cigarette store.

I hope this helps you.

... or just add one more battery "above" the transistor? If I'm right (check it!) that the ATtiny will be as happy on 4v5 as 3v, you don't even have to be "clever" about it... just run everyting at 4v5.

For a low voltage load you need a (logic-level) MOSFET, not a darlington.

There’s lots of problems the glow plug is 1.5 volt a AA battery is at it’s max rating to power this.
you add more you don’t get increase in amp’s just voltage. Plus you run into the problem of battery resistance.

His best bet is a relay to power the glow plug anything else is going to add lost to this.

This is workable

A relay isn't going to help shrink the device though, a MOSFET is much more compact.

These are really big lol about 3/8 by 3/8 by 3/4 long small

I don't think you need so much heat to pop a balloon. Have you considered a smaller heating element? If it is disposable, you could use thin aluminum foil strips as micro fuses. Once a small hole is open, it quickly gets bigger.

I suspect that good physical contact between balloon and the thing getting hot is a big issue here... but also, that if tape is used to hold one against the other, care will be need to be taken in how it is attached, or the tape may impede tearing of balloons fabric! (Also... something "outside" the heating element, to stop heat loss may help. (I.e. create a "sandwich": balloon/ heater/ thermal insulation.)