Vespa 50 Special electronic turn signal relay

The original electro-thermo-mechanical one broke and a "new" one costs 80 eur+ - let's do a diy solution

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See "details" for Details ;)
Picture from FLCL. Parts ordered (except enclosure) from (shipping worldwide to everyone, paypal accepted)

0a. Electronic stuff is made of magic (smoke):

0b. Ok, let's draw it a bit more professional:

1. a bridge rectifier ensures the right polarity.

2. a resistor charges a capacitor, a voltage divider triggers a triac after some time:

3. the triac shortens the two terminals. this turns on the turn light(s) and slowly discharges the capacitor:

4. The cycle starts again beecause the "triac" does not fire for the supply pulses as the base voltage/current of the NPN transistor is too low.

Overall circuit (first sketch, no part values):

The design is related to this one. No triac is involved. Instead, two high-current bipolar transistors form a thyristor. The additional collector resistor on the base of the PNP /collector of NPN transistor pulls the base of the PNP transistor high, reducing the overall loop gain. If the circuit does not "flip", this is one location to adjust.

A quick prototype looks like this:

...and (I was surprised myself) works after adjusting the pot:

The flashing frequency/period does depend on the load on a tiiiiny amount: During the charge phase of the electrolytic cap the charging resistor (about 100k) is much larger than the load (a few ohms) which means the charging duration will be more or less the same. During the discharge phase, the "triac" sets a certain voltage which is more or less independent of the load, which results in a discharge speed which is more or less independent of the load. This is proven by experiment: A tiny light bulb results in the same frequency as 4Ohm load (=6V / 4Ohm = 1.5 Amps ; 9 Watt). The load resistors get reasonably warm while for the circuit itself it is hard to notice heat. Discharge time is not influenced by supply voltage, but charging time is. With rising supply voltage, on time stays approx. constant but off time reduces.

  • 4 × MBRS340T3 3A Schottky Diode for bridge rectifier
  • 1 × 22uF 63V low ESR electrolytic cap may be replaced with foil cap later
  • 1 × ZTX653 NPN 120V 2A 1W TO92
  • 1 × ZTX753 PNP 120V 2A 1W TO92
  • 1 × pcb Electronic Components / Misc. Electronic Components

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  • followup

    rawe01/20/2016 at 16:12 0 comments

    For the turn signals to be clearly visible on sunny days, it might be a good idea to upgrade to LED lights. Due to their non-linear current draw ripple on the supply make the circuit go crazy/flicker. A small additional load resistor and filter cap after the bridge rectifier do help.

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