This is an entry for the international 555 Contest. The 555 timer chip has been around for many many years and is extremely versatile.
The Concept of my project
Often in automation or electronics we need to be notified of an event while we are away.
This device was built to do just that. When the event occurs, it will call you on your cellphone immediately, and than hang up after 30 seconds (so that you do not incur any call charges). You should not answer the call of course, unless you want to hear whats going on on the other side. That could be useful, for example, you hook it up to your doorbell, and talk to the person who’s at your front door. Now they think you’re home and are unlikely to attempt a breakin!
It can be triggered by
- your doorbell at home
- your car alarm
- an infra-red motion sensor
- breaking a laser beam
- excess sound levels (baby monitor, security)
- your home alarm system
- a low water level sensor in your dog’s water bowl or your fish pond
- a wind sensor where you like to fly your kite/hang glide
- temperature/humidity sensors
The possibilities are endless! The only limitation is your imagination!
The device consists of a hacked cellphone, the 555 controller board, an LM317 power supply and opto-isolators to press the green and red buttons. Redial, Confirm, Hang Up.
The LM317 power supply makes the unit very flexible, it can run on any power source, (car battery, AC-DC converter, etc).
I built this device 9 years ago, using only 4 555 timers to provide all the logic needed to create an automated, edge triggered cellphone alert with automatic hang up.
I hacked the buttons by pulling the rubber/carbon keypad off, and soldering wires to each side of the button contacts. Taping up the areas you don’t want the solder to go helps!
After testing the phone and the connections I covered the buttons in hot glue to provide strain relief.
I have provided auxiliary access to the green and red buttons, so the phone can still be used to get it into the desired state, so thats its ready for automatic operation.
Basically you just need to dial the number that you wish to be called, then hang up, and its ready for action.
Even if the power is lost, the phone remembers the last number dialed.
Here is the circuit board. I constructed it on protoboard (the type that has lines). These days I prefer the protoboard with the separate circles.
The cellphone’s ringer was relocated to get all the components to fit more snugly.
Any home made electronics that goes in a car needs major strain relief to prevent parts from vibrating and breaking off.
Finally! Here is the schematic.
I’ve just digitized it from my hand drawn schematic from 9 years ago. I was quite careful, hopefully there are no mistakes. I’ll explain the operation below.
The top left corner is the power supply for the 555 controller electronics. It provides a regulated voltage to power the opto-isolators etc so that operation is reliable. The 555 is a very well designed chip, its designed to provide the same timed period regardless of voltage. However, quick voltage fluctuations would affect it, so its better to run them off a regulated power supply.
On the top right corner is the power supply for the cellphone. Lithium Ion batteries run at between 3.6V and 4.2V, so the phone will be happy with 3.725v, which results from the LM317 with the voltage divider of 240R and 470R.
Getting onto the good stuff!
There are 2 main configurations for a 555 timer. Astable and Monostable.
Astable means the timer runs and retriggers itself infinitely (unless it is disabled with the reset pin, or something else is manipulated).
Monostable operation is where the timer gets triggered by a low voltage on pin 2, the 555′s output pin 3 goes high for the timed period, then goes low again, and remains low until it is triggered again.
Firstly the 2N3904 provides a normalized trigger input for the system. Any positive voltage that can provide enough current will do the job. A resistor should be added inline according to the trigger voltage used.
All of the components on the trigger pins of U1 and U3 provide ‘edge triggering’.
This means that if the event occurs, and our input signal goes active, it can stay active without interfering with the normal operation of the circuit.
Without edge triggering an active signal would hold the timers on as long as the signal is present.
R3 provides a relatively strong pullup for the trigger pin, providing noise immunity. Without that, stray EMI would be picked up by the 555 and it would be falsely triggered.
C1 is a DC blocking cap. It will only allow a changing voltage to reach the other side.
When Q1 is off, R1 will ensure that C1 has no charge, its a ‘bleeder resistor’. It slowly bleeds off any charge stored in C1 so that its ready for action when Q1 turns on.
When Q1 turns on a negative voltage passes through C1 for a brief moment to the trigger pin, starting the timer.
When Q1 turns off, C1 will initially have a charge stored in it, negative on the left, and positive on the right. R1 will provide a weak positive voltage, in series with C1 so that the right side of C1 will have a voltage of VCC + V C1. That could easily damage the 555, so we shunt any voltages greater than VCC directly into VCC, which will easily absorb any charge stored in C1, with VCC voltage hardly making a blip. This works because the trigger pin is high impedence. (by the way, even if the charge in C1 was enough to raise VCC significantly, it still wouldn’t damage anything because VCC would still be higher than the trigger pin voltage.
C3, C6 and C9 all present a low resistance to ground when the circuit is initially powered up. They hold reset low for a brief moment, which inhibits the 555s from triggering.
This prevents any startup noise from falsely triggering them.
These capacitors are quickly charged up by R4, R8 and R10, pulling the reset pins high, so that the 555s are ready to be triggered.
C7 provides edge triggering for U4, in the same manner as C1. This causes U4 to be triggered when the bottom left timer ends its ‘ON’ period.
U1, U3 and U4 are all monostable. U2 is Astable.
OPERATION
Q1 turns on, current flows through C1 triggering U1 and U3 simultaneously.
When U1 turns on, it pulls the reset pin of U2 high, allowing U2 (astable) to run freely such that it presses the green button for 105ms, waits for 105ms, then presses the green button for 105ms, then would start waiting for another 105ms, but U1 turns off and U2 becomes inhibited.
U1 and U2 provide 2 green button presses in quick succession.
Now the cellphone will be making the call.
When U3 turns on, the positive output voltage from U3 in series with C7 makes a voltage greater than VCC, this is immediately shunted to ground, and C7 charges up, and then blocks the high DC voltage output of U3.
After 30 seconds, the output of U3 will go low, this change in voltage will pass through C7 and U4 will be triggered.
U4 is monostable, and gives the red button a long press; over 500ms. 500ms is not a particularly long press for a human to make. It takes over 2 seconds of holding the red button to turn the phone off. I just decided to be generous with the red button timing.
I made the green button timing quick to minimize the delay from the event occuring til the user’s phone rings.
Now U4 turns off and the process is complete.
Re-triggering
U4 is ready to be triggered again. U1 and U3 triggers are connected together, and C1 will have been ‘bled’ by R1 after all that time. U2 is always ready to be triggered because its Astable. Its low output pulls it’s trigger low. As soon as U2′s reset pin goes high it will start up.
Opto-Isolators
Opto-isolators are great devices for hacking stuff. They allow you to press a button, from a completely isolated circuit. You don’t have to worry about the interfacing voltages etc. You just connect the photo transisor (in the correct orientation) to the button, then power the opto-isolator’s LED. Both parts are internal.
REDESIGN: Single 555 timer with extra timers made from discreet analog electronic components!
Shortly after building this I came up with a radical new way of thinking about analog electronics. I started thinking up all kinds of ways to create delays and adjust sensitivity with just discreet components. It can be a bit tricky at first, but its very rewarding.
Then I redesigned the circuit to use only 1 555 timer instead of 4. The 555 is the heart of the circuit.
This was a simpler proof of concept test. It doesn’t have bleeder resistors, so operation is a bit hit and miss, but when it fires up properly, it does what its meant to do. Also note that theres no zener diode, so the green LED keeps lighting up dimmer and dimmer repeatedly. With a 2.4v zener in series, once the voltage drops below 2.4v its out!
Heres a video of the new 1x 555 circuit board in operation
If you look at the schematic you’ll see I’ve used a large resistor for R2 (1 million ohms) that is done to delay triggering. If the circuit is hooked up to a car alarm, you want it to ignore sounds where the alarm is being set or turned off, but when the siren is sounding continuously, you want it to trigger. So this can be adjusted depending on the application. The 2 transistors can also be omitted.
Operation of the Single 555 controller
This circuit is triggered by a low input voltage. It was intended to be connected to an opto-isolator which could then be connected to the siren of a car alarm. Little bleeps will be ignored (when alarm is set or deactivated) but the controller will be triggered by a continuous noise playing on the siren.
A low going voltage turns on Q1 just a bit, which slooowly charges C1. Once C1 voltage has gotten high enough, Q2 turns on and a low going trigger voltage reaches C2 which is in a discharged state due to bleeder resistor R4. The current flows through C2 and reaches the trigger pin of the 555. R9 pulls the trigger pin up again.
When the low input signal is released, R4 will apply a positive voltage C2, the voltage on the right side of C2 will be VCC + the voltage across C2. D1 immediately shunts this to VCC. R9 and R4 will keep C2 completely discharged.
R8 used to be 120k, which gave a period of about 29 seconds. But the cellphone networks have decreased the maximum ring time before voicemail. So I changed R8 to 87k, giving a ring time of about 21 seconds.
The usual R7 c5 combo inhibits the timer as it powers up (prevents false triggering on powerup).
When the output of the 555 goes high current flows through R5, the flashing LED, the Zener Diode, and the opto-isolator LED (for the green button) and through C3.
The Zener Diode is reverse biased, and will conduct as long as the voltage across it is greater than it’s breakdown voltage.
The flashing LED turns on and off at about 1Hz modulating the current through the opto-isolator LED which has the same effect as pressing the green button on the cellphone once every second.
The flashing LED in this circuit replaces U2 in the previous 4x 555 circuit. It acts as an astable current oscillator.
As C3 charges up, the voltage drop across it increases to the point where the voltage across the Zener Diode is less than it’s breakdown voltage, and current stops flowing through the flashing LED, the Zener Diode, and the Opto-Isolator’s LED. (it stops pressing the green button).
Without the Zener Diode the flashing led and opto-isolator LED would turn on many many times, with decreasing intensity. (we only need 2 pulses)
The cellphone doesn’t seem to mind if you press the green button 2x to redial (required) or 3 or 4x. If it was a problem then C3 would need to be made smaller, or a Zener Diode with a greater breakdown voltage would be required.
When the 555 turns off, C3 will be discharged through R5 and D3. (ready to run in future)
When the 555 turns on any voltage in C4 will be discharged by D4 and R6.
When the 555 turns off current will flow through R6, the opto-isolator LED (for the red button). As the voltage across C4 increases, the voltage across R6 will decrease which causes a decrease in current through the opto-isolator LED. The current quickly becomes so low that the opto-isolator’s photo-transistor will no longer conduct. This drop in LED intensity is demonstrated in the proof of concept video, it takes about half a second for the current flow to drop til the point where the LED is not visibly lit anymore. (represented by the green LED on the left)
A regulated power supply is espeially important when using discreet components to form timers in the ways demonstrated here.
Correlations between discreet components in the ‘single 555 controller’ and the 555 timers in the ’4x 555 controller‘
C3 acts as U1
C4 acts as U4
Flashing_LED acts as U2
The 555 performs the same function as U3
Conclusion
Interesting hey? There are always tradeoffs in design. This is noticable in electronics.
The 4×555 controller gives more exact operation, while the single 555 version seems to have a mind of its own, its own personality, quirks… its own nature! Thats what I love about it. Analog electronics has a lot of soul. This concept demonstrates whats so great about the 555.
Sure things can be done on modern cheap microcontroller chips these days with ease. Massive intelligence can be built into them, executed with clinical precision. With oscillators executing instrucitons millions of times per second, and many many lines of code...
But the hunble 555 is hanging in there! Its so beautiful and so simple.
I think the 555 offers a great opportunity for newcomers to learn about electronics, and particularly that some people can learn a few things from the techniques I’ve demonstrated here.
And if you’ve got any tricks to share with me, let me know 
Too often these days people join #electronics asking how to program a microcontroller to achieve the simplest of tasks, which could be implemented more quickly and reliably with discreets (theres no code to crash, less to go wrong). Some of these people don’t even know what a ground is or what a resistor does.
Its important to keep electronics-for-humans alive… Keep the hobby going, keep the barrier to entry low, keep parts manufactured in through-hole versions. The world needs more engineers!
I’m glad to see this competition running, I hope its a great success, and I hope to see the organizers running some more awesome analog electronics projects! (or anything really
Contests like this are actually awesome. They make fun and learning commercially viable and constructive.
Now people can justify taking the time to build something fun, because they may win some prizes, money or get something nice to put on their CV!
I think its important to give every entrant some recognition. Maybe a mention that they entered, and a link to their project.
Taking it a step further, allow people to give a thumbs up to the projects they like.
I hope you enjoyed reading this!
