Mailbox Alarm Ideas - Actual Mail Inside Mailbox

I put together a little logic circuit that tells me when we have mail in our mailbox and it has been working pretty good. I used two TCRT5000 proximity sensors and ran them through an op-amp and then to an OR gate, so if either sensor picks up the reflection of the mail, it would close the circuit on an Ademco wireless switch. The datasheet mentioned that the photo-transistor blocks visible light, but sunlight immediately trips the circuit. This really isn't much of a problem, because once the door is opened, then sensor trips the alarm and if mail is placed inside, once the door closes, the sensor sees the mail and stays in a tripped status. If no mail is placed and the door is closed, then the alarm will not remain tripped. I configured the alarm system to only chime & print "MAIL IN MAILBOX" on the keypads in the house, but to not activate the alarm in any other way. I chose this method, because I had the parts and I was bored during our COVID lock-down last year. I could not find a mailbox alarm that would notify me if there was actual mail inside. Most of them only alerted when the door had been opened. The whole reason behind this was to let my wife know the minute that the mail was delivered, so she could retrieve it as soon as possible. We routinely get heat-sensitive items and even during the Winter, the inside of the mailbox can get pretty hot.

This worked great until it started getting very hot outside (above 95 degrees Fahrenheit). Then the sensors get so hot that they trip the alarm sensor and the alarm panels chimes. This only happens briefly and the alarm sensor resets. Sense it only happens a few times during the hottest times of day, I just disabled the chime. We can still see the message on the keypads, but I'd like to get the chime back on, so she can hear when the mail gets delivered.

In hindsight, I should have used a selective IR sensor at ~38kHz, 1) to keep ambient light from tripping the alarm when the door is opened and 2) to keep the sensor from falsing. I'd like to redo the sensor circuit to only trigger if the photo-transistor receives a 38kHz signal from the IR emitter. Can a PIC be configured to look for a 38 kHz square wave on one of its input pins and then only output a high/low on a selected output pin if the 38kHz is a match for what is programmed? One of my problems is that I epoxied in the TCRT5000s to the top of the mailbox, so I'd like to retain the use of these sensors if possible. I don't mind buying another mailbox if the TCRT5000s just won't work, but if I can make it work, that would be great. Other than that, the rest of the circuitry is in a NEMA waterproof enclosure that is mounted to the 4x4 post.

Is there a "best" PIC for this application? I have several different types on-hand, but can order new if there is a best-case PIC for this purpose. The end goal is to trip the alarm sensor if either of the two TCRT5000s see a piece of mail without tripping on false readings. Bonus for allowing the circuit to be put to sleep and to remember its last state before it goes to sleep, then to wake when the door is opened. Any input is very much appreciated.

There are a ton of IR remote control receiver modules (Sony, panasonic, whatever) on the hobby market, through Adafruit, ebay, Electronic Goldmine, etc. Also, the TSOPxxxx parts are very available. Either of these gives you a logic-level output based on the presence of a 38 kHz IR beam. Add one 555 for the transmitter and you're done.

The parts can include an optical bandpass filter in the plastic, an electronic bandpass filter, adaptive detection threshold, etc. Sure, a PIC can do all of that, but why bother? Here is a typical datasheet. I'm sure people here and on other forums have "favorite" TSOP part.


ak

Thank you. That was what I was thinking with the parts I initially used. I figured that simple was better and it worked great all Winter & Spring. I wasn't thinking about heat making the IR sensor false. I do have some IR receivers on-hand, but that means I'll have to junk the mailbox and get another one.

rescue161 said:

I do have some IR receivers on-hand, but that means I'll have to junk the mailbox and get another one.

Click to expand...

Why?

Are you sure it's temperature and not the angle and intensity of the sun that's tripping it. Try adding a sun shade.

Mike.

AnalogKid said:

There are a ton of IR remote control receiver modules (Sony, panasonic, whatever) on the hobby market, through Adafruit, ebay, Electronic Goldmine, etc. Also, the TSOPxxxx parts are very available. Either of these gives you a logic-level output based on the presence of a 38 kHz IR beam. Add one 555 for the transmitter and you're done.

Click to expand...

Not one 555, either use two or use a 556 - you can't use a TSOP with a continuous carrier, it's got to be pulsed - otherwise they shut down and ignore the signal.

I though that he wants alarm when he got letter in email @. Yeiks.

The mailbox is sealed when the door is closed. so no sunlight is getting in. The original two sensors are epoxied to the inside top of the mailbox, along with the wiring that runs along the top. Both sensors have the emitter and the photo-transistor in each, so everything is on the ceiling and points down, looking for anything reflective. There is no way to get any of that out. The mailboxes are cheap at $18.. I don't like the idea of using a 555 as a timing circuit, because I'm sure that the temp changes will affect the frequency and it will probably cause more headaches.

rescue161 said:

The mailbox is sealed when the door is closed. so no sunlight is getting in. The original two sensors are epoxied to the inside top of the mailbox, along with the wiring that runs along the top. Both sensors have the emitter and the photo-transistor in each, so everything is on the ceiling and points down, looking for anything reflective. There is no way to get any of that out. The mailboxes are cheap at $18.. I don't like the idea of using a 555 as a timing circuit, bcause I'm sure that the temp changes will affect the frequency and it will probably cause more headaches.

Click to expand...

Easier, more accurate, and much more 'feature filled' to use a small PIC (even an 8 pin 555 sized one) - but using a pair of 555's wouldn't be an issue, as the frequency isn't at all critical, and a 555 is FAR more than accurate enough.

And, a sun shade?

Mike.

The genius of the 555 internal design is that the accuracy of an astable oscillator frequency is determined almost exclusively by the accuracy of the external components. At 38 kHz, 1% film capacitors and 0.1% resistors are pennies.

For a commercial product I'd go with a uC. But for a one-off, my guess is that if you're not a PIC wizard like Nigel, the firmware development total effort will exceed that of an all-hardware approach by a lot. A dual-555 pulsed oscillator circuit needs a grand total of 2 resistors and 2 capacitors to run, plus 2 more caps for power supply decoupling. That's not very much.

Where are you located?

ak

AnalogKid said:

Where are you located?

Click to expand...

What about when YOU put mail in the mailbox to be picked up?

Half the time, we have others pick up the mail and the mailbox lid isn't closed. They just don;t close the lid.

When letters are mailed from home, they are secured with a clothes pin and the lid is propped up about 1.5 inches

I finally got around to wiring up a fix. I went with an op-amp delay circuit, since I was already using two op-amps (one for each sensor). I changed out the dual op-amp for a quad and am able to adjust it anywhere from instant to 20 seconds. The cool thing about using this circuit is that the "timer" resets very quickly. Even a brief trip will reset the time completely, so I should be good to go. The alarm in the house won't chime until it stays in a tripped status for more than 8 seconds. It is really starting to heat up, so we'll see how well it does.

We always close the lid on our mailbox, especially when mailing something out. We don't want to risk losing our sent mail to damage from rain or loss from wind, etc by leaving the door open. We just raise the flag on the side so the carrier knows that we have mail to be sent. With outbound or inbound mail, the sensor will report that there is mail in the mailbox. I suppose I can add a sensor to the flag itself, so that the sensor won't trip if it is up. Or, since the alarm sensor has two switches, I could wire the other up in conjunction with the flag. I guess there are a bunch of ways that it could be done. Maybe I'll revisit it once I know this thing will work in the dead of Summer.

Also I don't think putting a sun-shade on the side of a busy road (55 MPH zone) would work.

Please post your final schematic.

ak