555 Turn Signal Flasher Circuit - Inconsistent Operation

[Village_Idiot]

Hi All, cooped at home during coronavirus, I decided devote my free time to making a turn signal circuit for my motorcycle.

The circuit mostly works, but I get strange behavior. In total there are three main elements to the circuit 1) astable oscillator 2) transistor NOT gate 3) Load sensor. The 555 oscillator feeds into a transistor NOT gate which drives the NPN output transistor. The purpose of the NOT gate is to ensure the signal lamp starts in the ON state. The oscillator and NOT gate work as expected. The element that doesn't work reliably is the load sensor. The purpose of this is primarily to minimize power draw when signal isn't being used (would drain battery when bike is parked), also it resets 555 (ie by powering it on) so indicator always starts in the ON state when toggled. See the schematic and attached LT spice simulation showing load sensor operation with load applied (D2, D3 current traces, C2 voltage trace).

In practice, the load sensor seems a little too sensitive. When load is applied everything works as intended. When load is removed or the circuit is powered on without load, the circuit will begin oscillating. The period of oscillations is longer than the 555's astable oscillation. I don't have a scope unfortunately, I'm just looking at the mA meter on my bench power supply. It shows a ~0.5 sec 60 mA draw, every 2 seconds. This eventually stops after 1 to 15 oscillations and the circuit stays off. I expected 1 oscillation when powered on without load, but continued oscillations are unexpected. Does anyone have any ideas how to make this circuit more reliable?

 

[Jesse18]

Is load the light connected to ground?

 

[gophert]

Your motorcycle battery is about 20 amp hours.
The icm7555 draws 50 microAmps quiescent current.
It will take about 4 years to draw 10% of your battery's energy.

a lead-acid battery self-discharges at about 5% per month so you won't notice the 555 timer on your bike.

life is much easier if you would get rid of the load sensor and just enable the flashing by pulling pin 4 high. Use a pull-down resistor to keep it off when your switch is off.

 

[Village_Idiot]

gophert, If the reset pin were pulled low, I agree the quiescent current wouldn't concern me, but when it's high this circuit draws enough to slowly drain a battery. I do agree, another strategy to accomplish this would be controlling the reset pin, but it's a pretty similar problem to the one I'm trying to solve here: bring something high when load is detected (Vcc or Reset).

jessie: Yep the circuit is sourcing the lamp which is connected to ground.

 

[gophert]

I guess I am not understanding what you are trying to do with your circuit.
Do you want to turn it on to flash and stop flashing when you stop pushing the button?
Or are you automatically turning off the bike's factory flasher after a certain time?
Something else?

your first sentance simply says, " a turn signal circuit".

 

[Village_Idiot]

The circuit always get's its respective 12V and GND connection, but when the turn signal is toggled (S1 in schematic) it sources indicator lights with a square wave causing them to flash. On top of that the other two things I mentioned in my initial post are key: light should always start in on state when switch toggled, and should use very little power when not connected to load by S1 to avoid draining battery when parked.

Anyways I was able to sort out the problem: C2 was very close to a GND trace on my board. After re-cleaning the board it's operating as intended. From modeling even a 2 Meg resistor parallel to C2 will cause the 555 to continue oscillating when disconnected from load. Looks like I discovered that first hand!

 

[rjenkinsgb]

You should always have a base-emitter resistor on any transistor switch, it's a fundamental of good design.

That is intended to bypass low level leakage through other components or the PCB due to contamination or humidity. It can be quite a low value as there can never be more than around 0.6V across it; eg. another 4K7 in your design.

Imagine what may happen in damp weather with than outdoors on a bike & some condensation on the board..

 

[dr pepper]

I remember reverse engineering a commercial unit, it was simpler, they did some trick to get the 555 to go high on power up, and the load sense was a shunt.
Usually the switch is a changeover switch inline with the indicator strings, this goes way back to when indicator relays were just a piece of bimtallic strips, I rmember those, all done with canbus nowadays.

 

[Village_Idiot]

rjenkinsgb: That makes sense I'll add that and include in future designs.

dr pepper: I was trying to think of a simple way to get the 555 to power up with output low! I couldn't come up with a simple way, guess that's why I don't get paid to do this

 

[eTech]

dr pepper: I was trying to think of a simple way to get the 555 to power up with output low! I couldn't come up with a simple way, guess that's why I don't get paid to do this

That's doable. Hold the 555 reset low when the chip is powered on. Question is: how long does the output need to stay low?

 

[dr pepper]

100u cap and a 100k resistor might be a good start working on breadboard.

 

[Village_Idiot]

Here's another strategy, basic idea is disconnecting Trig from 555 timing capacitor and pulling it low with a high value resistor in OFF state. This allows timing cap to charge up to Vcc and OUT to stay high. Not sure how much power a 7555 would use in this state. No plans to build this one, you guys just got me thinking

I finally got around to potting the circuit from the first post and installing into my motorcycle. Let's hope it has a long life!

 

[rjenkinsgb]

I was trying to think of a simple way to get the 555 to power up with output low!

Just connect the "other end" of the timing cap to power rather than ground.

 

[eTech]

Hello

Is this what you want?