555 monostable multivibrator with diode connected to TRIG pin

[NigthMoth]

Dear all

Please help me understand this unusual(?) 555 Monostable configuration in which 555's Trigger pin and trigger pulse source
connected together via reverse biased diode. There is a lot of explanations for similar monostable configuration without diode on the web, but I can't find any with that diode installed.

I found this type of circuit on page 161 in book "Mechatronics for the evil genius" by N. C. Braga, 2006. ("Project 17 - Robot with sensors").

At first, i thought this circuit couldn't work as monostable multivibrator, because D1(and D2) is reverse biased and I should mentally replace it with opened switch. This way there is no voltage on TRIG pin -> no voltage on 555's inner Trigger comparator inverting input ->
inner Trigger comparator is always HIGH -> 555's inner Flip-Flop's "S" is always HIGH.
And when THRESH pin > 2/3 V+, inner Flip-Flop's "R" also becomes HIGH and Fli-Flop goes to undefined state.
Moreover, closing "sensor" switch changes nothing: D1 becomes forward biased and connects TRIG to ground forsing inner Trigger comparator and inner Flip-Flop's "S" to HIGH, but they already HIGH and always HIGH because of reverse biased diode?

I built similar circuit (with transistor Led driver instead of relay) and it works.
But I not sure why.
I found that there is still 10-30uA current through D1 in both directions when pushing and releasing SW1 button.
I guessed that D1 acts like coupling capacitor and I tryed to change D1 to 0.001uF capacitor in my circuit, and this circuit with capacitor instead of D1 also works well.

So there are my questions:
1) Why and how this monostable multivibrator with diode D1 works and what is the role of D1 for it's performance?
2) If D1 acts as coupling capacitor, why not just use a capacitor instead of diode?
3) If it is known and common practice for some particular purposes to connect TRIG pin with diode , kindly provide me with links with explanations when such diode should be used (Googling doesn't help, or maybe I cant formulate search pharase in correct way)

Here is pictures of my circuit with details.

 

[betwixt]

The unmarked resistor keeps the diodes reverse biased but if either switch is closed, both diodes become forward biased by current from the trigger pin and both 555's are triggered simultaneously.

Brian.

 

[NigthMoth]

The unmarked resistor keeps the diodes reverse biased but if either switch is closed, both diodes become forward biased by current from the trigger pin and both 555's are triggered simultaneously.

Brian.

I understand this. I dont understand why there is need for this diode at all? And, as i understand, after switch became opened again, diode again became reverse biased and disconnect TRIG pin from pulling up resistor, so TRIG pin voltage is always less than 1/3 of V+ (or, maybe diode acts as coupling capacitor and pass some voltage pulse to TRIG pin when switch opens?)

 

[crutschow]

I dont understand why there is need for this diode at all?

I agree.
I don't see a need for the diodes.
Actually the diode would could be a problem since it leaves pin 2 floating when the diode is reverse biased and pin 2 should never be left floating.

 

[D.A.(Tony)Stewart]

This schematic has flaws with missing Notes and interface info on sensors and 555.

1. Never leave a high impedance input floating, unless the datasheet says it has an internal bias or pullup/down. e.g. TRIGGER is floating at Vcc/3.
2. Using the DPDT relay to force the robot in the reverse direction at full speed for RC time duration, (AFTER release of the wall sensor switch) essentially puts twice the voltage across the motor as the contacts arc with about 20x the rated motor current But it is effective cheap solution to prevent crashing into a sensed obstacle.
3. Look at the internal diagram of the 555 and see that the Trigger and Discharge are negative feedback in this case to a floating input, so expect the output to glitch low randomly as the floating input and gets noise while biased at the internal voltage of Vcc/3. ( hence eliminate diodes.)

 

[D.A.(Tony)Stewart]

After I simulated it , I realized the output =HI on the leading edge of trigger one-shot for momentary sensor Trigger =LOW
The output times out if the sensor returns open = HI
But if the sensor still active = LO after the 1-shot interval, then the output remains HI and goes LOW immediately after Trigger goes LOW.

Thus the Robot motor goes in the opposite direction for T ~ 1.3*RC duration for a momentary Trigger HI or until Trigger is removed, if > T .

my sim https://tinyurl.com/2amzhg9k