I’m unsure if this is the last, or the penultimate, or the penultimate-ultimate, eh who knows. Anyway….
So—as I was saying in the second part, I had to make two separate circuits. By doing so, I could have a dedicated power supply for the rocket igniter and one for the unnecessary circuitry. If you’re going to go out of your way to do something, you may as well overdo it!
After a chat with my friend, we decided on an adequately-sized box to house the circuitry, as well as 3/4 switches (each of different type), which need to be activated in a sequence. And with LED indicators, including flashing ones for more ‘over the top’ goodness, because you can never have too many LEDs.
Usually I would make a digital form of the circuit but I figured it’s a lot easier to draw this one (well, I say draw—it’s a rough plan, but it’s good enough!):
As you can see, even with all the added switches and flash circuitry, it’s still quite simple. There are three switches, all of which have to be on for the rocket to launch, as with the previous version. The difference this time is that two of the switches are DPST (Double Pole, Single Throw). A DPST is basically two SPST switches stuck together and controlled by a single mechanism. An SPST is the most basic of switches: it is either on or off. It has two terminals, which are either connected or not connected. Therefore, a DPST has four terminals. I opted for DPST switches because they allow me to control two things with one switch. This alleviates the problem in the previous version of all the LEDs shorting out.
The first switch is a key switch. Why? Why not? We made this to be as over-the-top as we could! So, to be able to use the launcher, you need a key. Turning this turns on the launcher, which is indicated by a green LED. The second switch is the arm switch, which has an obligatory switch cover to prevent accidental arming (again, not necessary, but over-the-top…). This switch also activates a blinking red LED, controlled by a 555 timer in astable configuration. A 555 in this configuration generates square waves, in which you can choose the mark-space ratio of the waves, or duty cycle. Below is the general schematic for an astable:
The rate at which it oscillates is dependent on R1, R2 and C. They affect the charge and discharge rate of the capacitor C. This was initially put on a breadboard to determine the correct “flash” rate. R1 and R1 were initially potentiometers, thus making it easy to adjust. Once we found the flashing rate we wanted, it was as simple as measuring the resistances and replacing them with a resistor of a set value. The value of C also affects the rate in the respect that a larger capacitor will take longer to charge and discharge. The final values I settled for were:
C = 100nF, R1 =330Ω , R2 = 10kΩ
Using those values, you can therefore calculate the ‘time on’ and ‘time off’. The formulae can be found here.
The final switch is a momentary push-on switch. The launch switch. The switch that makes things happen! This had a bypass resistor of 3300 kΩ. I designed this launcher so it’ll tell me when I have everything connected up properly. This is shown by a white LED, connected across the terminals of the arm switch, to act as a bypass when the switch is off. Using high resistances to short the switches ensures that enough current flows around the entire circuit—but not enough to trigger the launch of the rocket. Electricity always takes the path of least resistance, so activating the switches will bypass these resistors.
With all these factors decided, it was simple building the launcher. Below is the parts list:
- 1 x black ABS enclosure 150 * 80 *50 mm (£3.71)
- 1 x DPST key switch (£3.61)
- 1 x DPST toggle switch with cover (~£3.50)
- 1 x SPST momentary on switch (£1.39)
- 1 x veroboard 25 * 64mm (~£1.00)
- 1 x NE555 timer DIP8 (£0.44)
- 1 x DIP8 socket (few pence)
- 1 x BC548 NPN bipolar transistor (£0.15)
- 2 x ppp3 clips (£0.20)
- 1 x 4 AA battery holder (pp3 clip) (£1.19)
- 1 x two point terminal block (£0.10)
- 1 x white LED (few pence)
- 1 x red LED (few pence)
- 1 x Green LED(few pence)
- 1 x 100μF electrolytic capacitor (few pence)
- 1 x 10nF ceramic capacitor (few pence)
- 3 x 390Ω 0.25W resistor (few pence)
- 2 x 10kΩ 0.25W resistor (few pence)
- 1 x 3.3kΩ 0.25W resistor (few pence)
- 1 x 1kΩ 0.25W resistor (few pence)
- 1 x 330Ω 0.25W resistor (few pence)
The parts that are a few pence are usually bought in bulk, which makes them cheaper. The final price totals at around £16.29 (I added an extra pound to make up for the parts that are a few pence), excluding batteries. The items were sourced from Farnell and Maplin. Below is the finished product:
On the first launch using this new launcher, the rocket went up without a hitch. Mission accomplished! I don’t plan to stop there, though—the next version will have even more pointless features!