Electrical Firing Electrical Firing SystemsSystems

A design overviewA design overview

What we are trying to What we are trying to achieveachieve

Ignition of electrical igniters with safety and accuracy.

Reliability in all weather conditions.

Simple and foolproof operation.

Initial ChoicesInitial Choices Complexity/functions requiredComplexity/functions required

Power SupplyPower Supply

DurabilityDurability

FunctionsFunctionsNumber of cues

Hardwired or Microcontroller based

Internal/external rechargeable battery

Safety features such as key switches, dead mans handle

Modular system with several firing points

Continuity testing

Ask yourself what functions you REALLY need, and which are just nice additions.

Battery typeBattery type

<0.02--Lead Acid

10

-0.8-

9V (PP3)

5.1

4.01.50.5

12V

0.45Carbon-Zinc

0.35Ni-MH0.12Alkaline0.03Ni-Cad

AA 1.5VType

Typical ESR of a range of commonly used battery types:

(batteries in fully charged or new state)

ESR

Cell

Representation of a battery

Battery voltage, and why it Battery voltage, and why it mattersmattersResistance of a typical bell wire 0.17Ω per meter.

Take the case of 1 igniter 100 metres from the firing system:

17Ω * 2 for the bell wire + 1.5Ω gives 35.5Ω

12V/35.5Ω = 0.33Amps

According to the Davey Bickford product datasheet:

No fire current - maximum current fornon-ignition of the fusehead0.20 A for 10 s.

All fire current - minimum currentfor ignition of fusehead0.60 A for 3 ms

0.33A is in the ambiguous area where it may or may not fire. A 24V battery would double the firing current and therefore solve the problem

Hardware and Ingress Hardware and Ingress ProtectionProtectionIdeal cases are rated to IP68 – total protection against dust and

immersion in water.

Diving companies sell suitable casings but they can be very expensive. Prices start from around £40 but for larger cases can be over £300. Brands include Underwater Kinetics, Otterbox and Storm-Case.

A cheaper alternative is ABS boxes, designed for a range of electronic applications. They can be bought relatively cheaply however they are not usually watertight.

Safety FeaturesSafety Features

Key switch – key removable only in ‘off’ position.

Dead mans handle for automated systems – Stops all further firing when pressed

Continuity TestingContinuity TestingVital for all but the most basic systems.

Keep testing current as low as possible – 20mA as an absolute maximum. Low current LEDs are now available cheaply from any good electronics supplier.

To calculate the continuity test current, the formula is as follows:

(Supply voltage – LED voltage drop) Resistor valueWhere LED voltage drop is typically 2 for green, red and yellow, 3 for blue and white.

In this case 12V – 2V = 10V divided by 1000 gives 0.01 Amps or 10mA, well within the limits. If the power supply voltage is changed, do not forget to recalculate the new test current!

ConnectorsConnectorsSprung speaker connectors are quick to insert and are low cost. They are used by most firing system manufacturers.

FireOne uses a different type of sprung connector.

A basic circuitA basic circuit

A more advanced outputA more advanced output

Advice for microcontroller-Advice for microcontroller-based systemsbased systems

Do not make such a safety critical item your first project – Experiment with other things first until you feel confident with their use.

If using a network, ensure you are using a protocol that supports error checking. DMX does not support this. Something like RS485 would be ideal.

Don’t forget to add ESD protection and transient protection.

Are the timings going to be programmable via the PC? If not, you may need to add an LCD display to provide feedback to the user and verify timings are correct.

Any Questions?Any Questions?