Improve Your Useless Box

This instructable shows you how you can improve your useless box, when it is reacting way too fast so you have little time to remove your finger after toggling the switch to activate the box..

Step 1: Introduction

In the start or home position, the mechanical finger, that is connected to a gear motor M1, is pushing against micro switch SW1, so it is pushed open. That means it is switched off and not conducting. SW1 is located inside of the box.

The manual operated toggle switch SW2, that is on the outside of the box, is in the OFF position. When the user toggles SW2 manually to the ON position, the motor is activated and starts moving. The mechanical finger that is attached to the motor will move out of the home position and releases SW1, so SW1 gets switched on. The motor continues to run until the mechanical finger has pushed SW2 back to the OFF position. When SW2 is in the OFF position (SW1 is still switched on), the motor reverses direction and starts moving the mechanical finger back to the start position. When the mechanical finger has reached the start position again, it pushes SW1 into the off position. The current to the motor is now interrupted by SW1, so the motor stops and stays in the home position, until SW2 is manually switched in the ON position again.

Step 2: How It Works

State 1

In the home position, the mechanical finger pushes the microswitch SW1 open, so it is not conducting. The toggle switch SW1 is in the OFF position. The electrical circuit is interrupted and the motor does not get any current so it is not running.

State 2

Toggle switch SW2 is manually switched to the ON position by the user. Now current starts flowing through the motor and the motor starts running in clockwise direction. The motor moves the mechanical finger towards toggle switch SW2. As soon as the mechanical finger leaves the home position, microswitch SW1 is closed This has no influence on the current state.

State 3

The mechanical finger has reached toggle switch SW2 and pushes this switch into the OFF position. Microswitch SW1 is still closed. The motor reverses direction since the current is now flowing into the opposite direction. So the motor starts running counter clockwise, thereby moving the mechanical finger away from SW2 and back towards the home position.

State 4

The mechanical finger now has reached the home position again and pushes micro switch SW1 open, so SW1 is switched off. This interrupts the current to the motor and the motor stops. The mechanical finger is now back in the home position, waiting for the user to toggle SW2 in the ON position so the whole cycle starts over again

Step 3: How to Improve (delay Circuit)

After i assembled the useless box and tried it, i found that the motor was moving way too fast. So when you toggled the switch, the switch was pushed back to the initial state almost instantly before you had the chance to retract your finger.

To solve that problem, i added the the following circuit that causes an adjustable delay. This delay prevents that the motor starts moving right away so you have the time to move your finger out of the way. The delay is created by capacitor C1, which is charged via R1`at the moment that SW2 is toggled to the ON position manually by the user, connecting the motor to the circuit. At first instant, C1 is not charged, so the voltage over the base-emitter junction of the darlington transistor Q1 is 0 and Q1 will not conduct. When C1 is charging and the voltage over C1 reaches about 1.2V, the darlington transistor Q1 will start conducting. With R1, the delay can be adjusted, because R1 determines the charge current for C1. If you want a longer delay, the 5K potmeter R1 can be replaced with a 10K potmeter to get maximum 2 seconds delay. Or you can double the value of C1 to 2200uF, but that might get too bulky to fit into the box. I used a darlington transistor to minimize the base current in order to minimize the load that the transistor forms on the RC network. Darlington transistors have a very high beta, i.e. current amplification = ratio of the collector current and the base current. A logic level P-MOSFET can also be used because it has a low gate voltage threshold (1 to 2V). A standard P-MOSFET has a gate threshold voltage between 2 and 4V, so it is useless for this circuit since it is powered with 2xAA batteries = 3V. Furthermore a MOSFET will not load the RC circuit, because its gate is voltage driven. With this circuit in place, the motor is not started instantaneous when SW2 is toggled by the user, but is delayed by a period that is set by R1 (between 0 and about 10 seconds).

Step 4: Pictures

Step 5: Video

Comments

[HarryFlatters]

Hi,
Many thanks for the circuit.
Should this work with a 3v supply (my useless box only uses 2 x AA)? I have made the circuit, but the motor now only turns very slowly.
Also, I am not an expert, so excuse me, but does you diagram imply that all 3 connections of R1 are made? It seems to suggest that the wiper is connected to one of the other pins. I have tested with 2 and 3 pins connected with the same result.
Many thanks again :-)

[Roel Arits]

About the 3 connections of R1: You can use 2 connections only, namely the wiper (middle) and one of the outer connections. It does not matter whether you use all 3 connections or just 2.
About the slow motor : I started with 2x AA batteries as is stated in the schematics, but afterwards i changed to 3x AA batteries as you can see in the pictures because i wanted the motor to run a bit faster. The reason for the lower speed (when compared with just connecting the 2x AA directly to the motor) is that the darlington transistor drops about 1V (collector to emitter voltage). This 1V drop is inherent to darlington transistors and can not be changed. This makes the motor run a bit slower than without the circuit installed.
I wanted to keep the electronics as simple as possible and using easy obtainable parts. There are better solutions to create a delay, but then you need more components.
Simple circuits always have drawbacks and compromises ... but they are easy to make. :-)

[HarryFlatters]

Hi Roel, many thanks for the swift reply. I understand your answers, glad they weren't totally daft questions!
I have taken to electronics very late in life, but find it fascinating. I am nowhere near being able to design my own, even simple, circuits. Just trying to understand the use and purpose of components.
With regard to the delay circuit, ideally, what I would like is a circuit that provides a random delay of between 0 and say 3 seconds. I appreciate that this kind of thing could perhaps be achieved using an Arduino (or similar), but for me that defeats the understanding of how a circuit like that actually is constructed and works. Would you know how to go about designing such a circuit? Many thanks again. Steve

[Roel Arits]

Hi Steve,
Well, i learned the most by trial and error and by trying to understand existing circuits of other designers using books, magazines, service manuals, internet. This gives you both insight in how components/circuits work and also shows you different approaches to solve the same problem. You learn that there are many ways that lead to Rome. Selecting the way you want to go depends on cost, complexity, how accurate, how flexible a circuits needs to be.
If i think about timing or generating delays, i think of the 555 timer chip. This chip is an old and very flexible workhorse that was first produced in 1972 and is still used a lot. The timer can be configured as a continuous running timer, but also as a one-shot (monostable multivibrator) by adding resistors and capacitors. But we want the circuit to run from 3V, and that poses a problem : the power supply range of the 555 is 4.5V to 15V, so it can not run from 2x AA batteries. But there is a CMOS version of this chip, namely LMC555, that runs from power supplies down to 1.5V. So that is a candidate. Next we check the application notes for the chip to see how to configure it as a one-shot: when the circuits gets powered, it has to wait for a while before the output becomes high. The next thing to solve is : how to control the motor. The LMC555 output can not drive a motor directly, since it can deliver max. 100mA, which is not enough to drive a DC motor under load. So we need a power transistor that is able to deliver that current. This is the kind of think process that i would follow for a better circuit that is more flexible and not too complex. An Arduino is of course also a solution, and a solution that a lot of hobbyist would choose, because of the low cost and it doesn't require much knowledge of analog electronics. But to me an Arduino is overkill for such a simple task. It is almost an insult to the microcontroller to have it control just 1 pin to generate a delay. :-) But that is because i like to work with analog electronics that can also be build be hobbyists
that are not into programming in C.
Another route i would try to to use the same circuit, but then with a P-MOSFET with a very low gate threshold voltage (as low as 1V). The problem with that route is that these kinds of MOSFETs are harder to find/obtain. That makes this route less attractive for hobbyists because these components will not be found in electronics web-shops.
So i think i would go for the LMC555 having some experience with designing analogue electronics. Otherwise an Arduino Pro Mini would be the next choice, because it's cheap and you don't need experience with analogue electronics. You can drive the motor via a relay module (incorporating the relay driver). Otherwise you will still need a power transistor (with a maximum collector current of lets day 1A) to drive the motor directly.

I hope this helps a bit.
Cheers,
Roel.

[HarryFlatters]

Hi Roel,

Many thanks for such an in-depth response.

As you say "trying to understand existing circuits" is a great way to learn. To this end, would you be prepared to design my random 0-3 second delay module, using whatever components you deem to be most appropriate for the job?

Cheeky request, I know, but a s Mum used to say, If you don't ask......

Anyway, many thanks for all your help.

Kind regards

Steve

[Roel Arits]

I don't have a lot of free time to spend, but when i have a few hours, i will give it a go.
Best regards,
Roel.

[HarryFlatters]

You are a star - thank you :-)

[Roel Arits]

Where do you live ? I ask because in the US f.e. the 2Nxxxx transistor types are more common then the BCxxx that i use a lot because they come from Philips Europe.

[HarryFlatters]

Hi Roel, I'm in the UK. BC does seem to be more popular here too, than 2N.

Kind regards

Steve

[Roel Arits]

Hi Steve. On special request and because moms are always right, i took some time designing and testing a better solution for the delay circuit.You can find the circuit and some explanation about it here:
https://sites.google.com/site/roelarits/home/projects/misc/useless-box

Cheers,
Roel

[HarryFlatters]

Hi Roel,
I knew you couldn't resist (see what I did there!) the challenge. :-)
I've seen the improved circuit on your Google site, and will shortly go try and produce a PCB for it.
Many thanks for taking the time to design this. I realise that compared to some of the other stuff you have done (just spent 2 hours looking at your other projects!), this is not a difficult project, but it is ideal for a beginner like me to study and get a better understanding of some basic principles and will hopefully inspire me to go on and design circuits of my own.
Thanks again! I will report back once I have the PCB, and built and tested the circuit.
Best regards
Steve