Simulator Aircraft Gear Indicator Lights

A Gear indicator is a small touch of authenticity to Flight Simulator experience but I did not want to get complicated or expensive so, when I thought of the sequence "Red" - in transit, "Green" - locked, the "Larson Scanner" came to mind. The "Larson Scanner" kits can be bought for less than the cost of the individual components and so, with a little modification I offer the following design.

This is intended as a true "Instructable", giving design notes and a TinkerCAD example:

https://www.tinkercad.com/things/edEheplgdgz-gear-indicator

Name Quantity Component

   UIC1 1 Timer LM555

   UIC2 1 Johnson Decade Counter CD4017   

   DLED1    DTest 2 Red LED    

   DLED2    DLED3 2 Green LED    

   DD1, DD2, DD3, DD4, DD5, DD6, DD7, DD8, DD9

9 Diode 1N4148

   RR1    RTest 2 1 kΩ Resistor    

   RR2    RR6 2 470 Ω Resistor    

   RR3 1 10 kΩ Resistor    

   RR4    RR5 2 4.7 kΩ Resistor    

   CC1    C1 2 100 nF Capacitor    

   CC2 1 330 uF 16 V Polarized Capacitor    

   SReset 1 Pushbutton    

   JPConnector 1

The idea is to use the "555" to clock the 4017 ripple counter but use the Clock inhibit input of the counter fed from the 10th output to stop the count.

Of the ten outputs then the 1st to 9th are available so if "Green" is connected to the 9th then "Red" can be paralled 1 to 8 with Diodes for the transit time. A little more realism might be obtained by using the 8th count for a nose wheel, followed by the 9th plus 8th for all.

The Reset input clears all outputs and allows the count to 10 to re-run. My requirement is for a momentary switch contact to 0 volts so I will have to add a transistor inverter.

As the counter dis-ables itself then the "555" is left to free run in astable mode so the main design tasks are about timing and the LED drive.

Please note that the TINKERCAD image is mis-leading as the Counter is a 5 volt version whereas the normal device is a CMOS 15 volt type; also the LEDs may require extra drive power as the CMOS output is quite low.

Reset is normally "low" for operation so I choose a 10Kohm Resistor as "pull-down"; The CD4017 is a CMOS device requiring very little input bias but a high value "pull-down" could cause interference problems and too low a value is un-necessary, but 10Kohms is enough to keep the switching clean. The circuit has to make a negative switch operation into a "high" Reset so it is capacitor coupled to the positive voltage: This has the advantage of making a reset at Power-on. 

I am designing for a supply of 5 volts though it may be used up to the allowable maximum 18 volts meaning 5 volt or 12 volt computer supplies are acceptable.

1. The clock rate for 9 counts at n seconds for transit time.

2. The trigger for Reset.

3. Light Emitting Diode current

1a.The maximum clock is 2.5 megahertz for the 4017 and 5 Megahertz for a LM555 so this determines the minimum pulse width of 200nSec.

1b.The total clock period is to be n seconds divided by 9. If the transit time is taken as 27 seconds then the clock period is 27/9=3 seconds.

From the LM555 datasheet, T = t1 + t2 = 0.693 * (RA +2RB) * C gives the Period and so, using 330uF and 4700 ohms gives 3.23 seconds (The values are not important as long as they fall inside the recommendations but note that a higher value resistance has the benefit of using less power and the capacitor tolerance becomes less important i.e 33uF and 47Kohms).

The duty cycle is not important so I use the same resistor value but the capacitor will have to be an Electrolytic type which typically have very large tolerances: The actual time could be 3 to 7 seconds!

2a.The Reset minimum pulse width is 250nS. The Reset input needs to be set "high" i.e greater than 3.5 volts so using a 10Kohm pull-down resistor then a capacitor of 25pF would be a minimum.

In reality, 0.1uF capacitor would be more practical giving t=CR of 10Kohm * 0.1uF = 1mSec to 0.7 of 5 volt i.e 3.5 volts: That time is not quite enough as an extra 0.25mSec is required for Reset so something more than 0.1 uF is needed.

3a.LED current will depend on the diodes used; note that different coloured LEDs have different forward voltage drop which becomes more pertinent when driven from only 5 volts.

The RED LED is connected via diodes to outputs 1 to 8 giving a total of 2.6 + 0.7 +0.4 = 3.8 from 5 volts = 1.2 volts for 1mA or 1.6 volts using internal current limiting for nominally 3mA.

At 2.4 forward volt drop for GREEN LED it is marginally better but not if they are paralled 2 or 3 so a transistor driver may be needed.

Here you can see I used a "Larson Scanner" p.c.b kit plus a few diodes. The extra p.c.b in the picture holds a transistor driver for the two Green LEDs and the Reset circuit.

The final picture shows "All Greens" and the loose wire is Reset. The picture is a bit cramped because I used some Christmas LEDs and the wire was not very long!

t=CR is accepted to be the approximate time to charge to 0.707 of full voltage and so is treated as a roughly linear slope; 5CR being nearly full charge and clearly non-linear.

At power-on, the "555" has to charge the first 0.33 of a charge cycle to get past the normal trigger point of 0.33 so it would be ideal to hold the counter in Reset for that time but it is not important in this design, in fact, worse, also the counter will run a spurious count at power-on until it reaches the self inhibit count.

The T=CR time of the "555" is 2(4700 ohms) * 330uF = 1.55 seconds i.e to 0.7 peak and so about 1.55/2 = 0.77 to 0.35 peak.

The CD4017 data sheet gives the output current at specific supply voltages which is given as typically 1mA for sink or source for an internal voltage drop of 0.4 volt. If the LED forward volt drop is 2.6 volts then adding the 0.4 gives 3 volts so the external resistor must be (5-3)/1mA i.e 2Kohms but this implies a very high efficiency LED!

The data sheet shows internal current limiting so, if package heating or 100mW per output is not exceeded then no resistor is required!