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edited Mar 13, 2021 at 11:21

7408 quad-NANDAND with diode latching: sporadic (and undesired) self-latching at power up

As an introduction to electronics, I am still working my way through Charles Platt's Make: Electronics (2nd edition).

This issue is about Experiment 21: A Powerful Combination

In circuit on figure 4-107, a 7408 (quad-NANDAND) is used to implement a simple switch based "password system". The different switches have to be pressed in a given sequence, each switch allowing to change the output of a NANDAND gate to high, thus latching this NANDAND and providing a high input to the sub-sequent NANDAND gate.

(I suppose I am not allowed to post the circuit schematic from the book, as it might be copyrighted material?)

Minimal example

I have been having problems with the resulting circuit sometimes starting in the "unlocked" state, without having to first press the correct sequence of switch. By fault tracing with a voltmeter, it appeared that, sometimes, the different gates are latched in high-output at circuit startup.

Here is a minimal example showing the latching for a single NANDAND gate:

And here is the corresponding schematic (my first, please be indulgent):

schematic

^{simulate this circuit – Schematic created using CircuitLab}

About 1 in 5 times when applying 5V to the power bus, the low-power yellow led will light up, without having to first press the switch.

Questions

What could cause this undesired (and erratic) self-latching at startup?
Is there a design flaw in this circuit? (or is there an obvious wiring error in my attempt to build it?)
The book is using 74HC08, where I am using 74HCT08N. Can it lead to the problem?

7408 quad-NAND with diode latching: sporadic (and undesired) self-latching at power up

As an introduction to electronics, I am still working my way through Charles Platt's Make: Electronics (2nd edition).

This issue is about Experiment 21: A Powerful Combination

In circuit on figure 4-107, a 7408 (quad-NAND) is used to implement a simple switch based "password system". The different switches have to be pressed in a given sequence, each switch allowing to change the output of a NAND gate to high, thus latching this NAND and providing a high input to the sub-sequent NAND gate.

(I suppose I am not allowed to post the circuit schematic from the book, as it might be copyrighted material?)

Minimal example

Here is a minimal example showing the latching for a single NAND gate:

And here is the corresponding schematic (my first, please be indulgent):

schematic

^{simulate this circuit – Schematic created using CircuitLab}

About 1 in 5 times when applying 5V to the power bus, the low-power yellow led will light up, without having to first press the switch.

Questions

What could cause this undesired (and erratic) self-latching at startup?
Is there a design flaw in this circuit? (or is there an obvious wiring error in my attempt to build it?)
The book is using 74HC08, where I am using 74HCT08N. Can it lead to the problem?

7408 quad-AND with diode latching: sporadic (and undesired) self-latching at power up

As an introduction to electronics, I am still working my way through Charles Platt's Make: Electronics (2nd edition).

This issue is about Experiment 21: A Powerful Combination

In circuit on figure 4-107, a 7408 (quad-AND) is used to implement a simple switch based "password system". The different switches have to be pressed in a given sequence, each switch allowing to change the output of a AND gate to high, thus latching this AND and providing a high input to the sub-sequent AND gate.

(I suppose I am not allowed to post the circuit schematic from the book, as it might be copyrighted material?)

Minimal example

Here is a minimal example showing the latching for a single AND gate:

And here is the corresponding schematic (my first, please be indulgent):

schematic

^{simulate this circuit – Schematic created using CircuitLab}

About 1 in 5 times when applying 5V to the power bus, the low-power yellow led will light up, without having to first press the switch.

Questions

What could cause this undesired (and erratic) self-latching at startup?
Is there a design flaw in this circuit? (or is there an obvious wiring error in my attempt to build it?)
The book is using 74HC08, where I am using 74HCT08N. Can it lead to the problem?

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asked Mar 13, 2021 at 11:08

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7408 quad-NAND with diode latching: sporadic (and undesired) self-latching at power up

As an introduction to electronics, I am still working my way through Charles Platt's Make: Electronics (2nd edition).

This issue is about Experiment 21: A Powerful Combination

(I suppose I am not allowed to post the circuit schematic from the book, as it might be copyrighted material?)

Minimal example

Here is a minimal example showing the latching for a single NAND gate:

And here is the corresponding schematic (my first, please be indulgent):

schematic

^{simulate this circuit – Schematic created using CircuitLab}

About 1 in 5 times when applying 5V to the power bus, the low-power yellow led will light up, without having to first press the switch.

Questions

What could cause this undesired (and erratic) self-latching at startup?
Is there a design flaw in this circuit? (or is there an obvious wiring error in my attempt to build it?)
The book is using 74HC08, where I am using 74HCT08N. Can it lead to the problem?

nand fault latching