Driving a two-aspect signal head
Driving a two-aspect signal head is straight forward - set the accessory decoder to continuous operation and connect each aspect to the output as usual.
Driving a three-aspect signal head
Set the accessory decoder to provide a short burst on the output (say, half a second long). Then use a latching circuit like that shown below. When the accessory decoder switches an output 'on' it actually provides ground, so this circuit starts the latch on a falling edge but does not start the timing until the rising edge. If a second rising edge is seen before the time is up, the elapsed time is reset to zero. So by pulsing the accessory decoder's output we obtain a constant output. Changing which output is pulsed will immediately change the latch.
Because of the latches there is a delay in switching between certain aspects, but this isn't necessarily a bad thing - it gives a bit of randomness and keeps drivers on their toes!
This table summarises the transition delays:
| Transition |
|---|
| From | To | Delay |
|---|
| Red | Yellow | No |
| Red | Green | Yes |
| Yellow | Green | Yes |
| Yellow | Red | No |
| Green | Yellow | No |
| Green | Red | No |
You'll want to use (write?) some software to update the states of the signal head by refreshing the accessory decoder outputs at least as often as the monostable resets (the timing is relative to the 1.5MΩ resistor and the 10µF capacitor - these values give approximately 16.5 seconds from the rising edge)
Circuit diagram created using the rather good Yenka Electronics package.
- Terminal 'C' is the common high
- Terminal '+' is a switched low
- Terminal '-' is a switched low
- Terminal 'L' is the common low
Parts list
I used parts from Maplin. Each signal head requires:
- One 820Ω resistor - M820R
- Two 22kΩ resistors - M22K
- Two 1.5MΩ resistors - M1M5
- One 556 timer chip (two 555 timers in one package) - QH67X
- Two 10µF electrolytic capacitors - VH22Y
- Three IN4001 diodes - QL73Q
- Three 3mm Green LEDs - WL33L
- Two 3mm Yellow LEDs - YY38R
- Two 3mm Red LEDs - WL34M
I personally prefer the HE red LEDs (rather than SB) - to my eyes they match the brightness of the other colours better. The circuit diagram shows two separate 555 timers rather than the single 556 package for simplicity's sake.
Circuit Design
The circuit contains two simple monostables, each having the reset pin driven from the opposing input, ensuring that in normal operation a maximum of one monostable will be active (though in some circumstances when power is initially applied, both monostables may start running, but the circuit will reach a stable state after the latching time period.) Two LEDs are used for each of the timer outputs to protect the LEDs from too high a reverse voltage when the other timer output is active. Three green LEDs and diode provide such a high-cost path that the green LEDs light only when neither of latches is active. I keep the 'spare' LEDs on the circuit board, but they could perhaps be used to provide advance or distant signals.
Lenz LS100/110 accessory decoders
Connect as shown in the circuit diagram.
Hornby R8247 accessory decoder
Voiding your warranty
The Hornby accessory decoder does not provide the common low as a screw terminal, so you need to access the PCB (there are screws under the four rubber feet) and connect a flying lead.
The common low is provided as the circular solder pad to the right of each output's screw terminals (but check your board with a multimeter first!).
The common highs and lows are common across all four outputs of a single accessory decoder, so you need only attach one flying lead to power up to four signal heads.
Connecting to the circuit
Connect as shown in the circuit diagram, using the flying lead for terminal 'L'.
7 May 2009.